When you shoot someone with a gun like the Tri-Beam Laser Rifle, is critical chance calculated just once for the shot, or for each of the three beams separately? I'm finding that it seems to crit almost every time I fire it (with Luck 10, Finesse, and 1st Recon Beret)...

For all separately, as far as i know. That's what made the Metal Blaster from the Pitt in Fallout 3 damn powerful.

Well, technically what made it so powerful was not that each projectile checked separately, but that each pellet on a successful crit would gain a critical damage bonus that was disproportionate to the damage of that projectile. For example, the Terrible Shotgun did 80 damage per "shot" - roughly 8.9 damage per pellet with 9 pellets. However the critical damage bonus of this weapon is 40, and each pellet would gain that much of a bonus, meaning if a pellet crit it did 48.9 damage on its own. Combine that with a sneak attack (which does twice as much damage as a normal critical) which would guarantee that each projectile would critical, you wind up with some crazy total damage. I believe they balanced the critical damage bonus around VATS, which will only allow one critical strike to occur per usage of AP (per shot), so if you do crit with a multi-projectile shot only one of those projectiles could gain the crit damage bonus and the others would not. Unfortunately this does not apply to non-VATS shots leading to some rather broken damage scenarios.

This has been rebalanced for New Vegas (and rightly so), so that each pellet will gain a critical bonus proportionate to the base damage of the pellet itself. For example the Tri-beam Laser Rifle deals 66 damage per shot - 22 x 3 - but the critical damage bonus is 22 to match the damage of an individual beam.

Not exactly... in fonv if your in vats all the shots get calced as one but in rt each shot gets calced seprately. Thus shotguns and the tribeam and multiplas do better in rt then in vats. The multiplas for instance does 105 crit damage so if even 2 of its blobs hit thats quite nice in rt.

This explains a lot. Thanks!