The wording in the house rules is very clear-not to pick the highest number of the three for luck, but the best for your character, and when new players ask why anyone would want an intentionally low luck, the older players always say just one word: Swashbucklers.
[If you're familiar with the swashbuckler luck mod, stop reading here, and pick up at the next purple text.]
Swashbucklers are a mod in which a player (usually, but not always a rogue) voluntarily gets a bonus to every action roll (skill, to-hit, etc.) in one set of situations in exchange for taking a penalty to every roll in the inverse of the situation. The exact nature of the mod is dependent on how the character is created, but usually follow one of two scenarios, the heroic swashbuckler or the environment-based swashbuckler.
The heroic swashbuckler gets the bonus when doing things that are selfless (or at least don't mostly benefit himself), and the penalty for doing things that are selfish (or mostly benefit himself.) The environment-based swashbuckler gets the bonus when doing things in his 'comfort zone,' and is a complete screw-up when he leaves it. The most frequent form of this environmental mod is on land and on water, so a swashbuckler might be suave and dexterous when climbing the rigging of his ship, then fall flat on his face and stammer in a courtly (or other on-land) situation.
What's important to remember is that no one is required to take this modification. It is a voluntary thing for people playing characters with the narrative of being very much fortune's fool. I allow players who've chosen the modification to drop it at will (however, they can only get it back by voluntarily dropping one level of experience or a permanent point from any stat, which I've had happen) and I've allowed a party to use a cleric of the god of fortune to cancel out a character's luck mod when it began to make the party crazy.
Before I explain the benefits of the swashbuckling luck modifier, let me add that most of the times I've used it, it's been for NPCs. It can get exhausting as a DM to try to take it into account, and if you have 2 characters with the mod in the same party, it gets insane. So let me explain how it works.
Never a dull moment...
When the character is doing the thing that he gets the bonus for, percentiles are rolled in addition to every normal roll that represents something he does. The DM knows these percentile rolls, but the player does not. When he rolls his luck score or under, the numbers on that die should be considered in the best possible light for the character. So, for example, the heroic swashbuckler trying to romance the jailor's wife to get the keys to free his party (a selfless act, in this case) who rolls his charisma to sway her into his arms doesn't merely succeed, but instead has her throw her arms up in the air in despair, finally talking to the one man in the world who understands, only to have the keys fly from her hand into the water bucket being brought to his party, which the swashbuckler notices as he sweeps her off her feet, but the servants carrying the water do not. In short, any success should occur in a dramatic fashion when the percentiles are on his side, and should occur in a normal fashion when the percentiles are not. Any failure should occur in a normal fashion.
When the character is doing the thing he gets the penalty for, percentiles are also rolled in addition to his check, with the same luck score, only now it is essentially an 'unluck' score. So, having freed the party by romancing the jailor's wife, the heroic swashbuckler decides that, since he's getting lucky, he might as well take advantage of the jailor's wife, and he proceeds to make mad, passionate love to her, rolling to see if he gets his...ahem...business taken care of before the jailor comes home and slays him. If he rolls his luck score or lower (just as in the first example) while doing this 'selfish' thing, he should be assumed to get the worst possible result if he fails. So if he manages sneak out of the window of the jailor's wife when hubbie comes home unexpectedly, he might find he left his lute with his name on it in the room, or end up hanging by his fingertips from the window naked just as the girl he's been trying to convince to marry him is walking by. Successes are treated normally, but failures are dramatically awful when the percentiles are against him. A player can make the unluck situation a rare scenario by taking a voluntary low luck score at creation. Remember, luck can't make you fail or pass the stat check, it can only make the failure more spectacular or the success even better.
In this case, a high luck means a high chance of stunning successes and of devastating failures (and a slightly improved chance of surviving a rare automatic death scenario.) A low luck means a low chance of stunning successes (but still the normal chances of success) and a low chance of devastating failures (and a fairly normal chance of surviving a rare automatic death scenario.) Lastly, a luck as close to 50% as possible means you've got about the same score whether you're trying to succeed or fail. When you bear in mind that a swashbuckler can determine his bonus times and penalty times at character creation (always with the DMs permission) the player can game the system fairly easily ("Oh, the captain? He never leaves the ship, not for a second! He thinks things never go his way when he's on land!") to make the character he wants.
Swashbucklers can become tortured souls, afraid to step outside of their comfort zone because it always goes wrong when they do, which can be a very fun experience for a DM, especially with an NPC. What's important is to remember that for a swashbuckler there is never a dull moment, they are a study in contrasts, and writ large, and this can be used to drive a party to the brink of insanity if played well, which is why it's a mod that exists in the fine print, even if it is hardly ever used.
[Continue here if you don't care about the description of the swashbuckler luck mod.]
When my players first met Sir Four as an NPC, it was an enormous series of dramatic rolls that created the narrative that he was a sort of swashbuckler. He pretty much rolled 1s and 20s constantly for his entire time with the party, rolling at least one critical hit or fumble in every battle, and usually rolling both in every protracted battle. (For example, he waded through a group of trolls, cleaving heads and body parts, only to have his holy sword fly from his hand and embed in the wall right before facing the bloodthirsty evil cultist who commanded the trolls.) In one battle, he landed an enormous hit on a giant just before the giant sent him flying across the room, which led to the recurring theme of giant-sized enemies throwing Sir Four around the room. (Sir Four is my favorite NPC right now. He yells...all the time....even when not appropriate or happy.)
When Sir Four returned to the party, I added the swashbuckler luck to his bag of tricks, ruling he gets the bonus when doing things that are super paladiny, like holding a chaotic evil beastie at bay, and that he gets the penalty when fighting things that are only mostly evil or when doing 'normal guy' stuff, this essentially allows me to use the piety stat rubric without having to actually keep track of piety. (Piety is essentially a measure of how well a deeply religious character is living up to his deity's expectation of him, which can be very important for some clerical orders. It is one of a handful of behind the scenes concepts/numbers I keep on characters in a campaign.)