The Inquisitor's Handbook appendix has a listing of all the guns in the DH core and IH, sorted by type; I'm just running off stats listed there, under the assumption that if you're fighting guys with TB 2 and armor 3 then anything from Ascension or Rogue Trader is out of your league.
Armor 3, TB 2 is actually pretty weak - even your average Guardsman has armor 4 all locations and a TB of at least 3. Having said that, lasguns are mostly good for having lots of ammo, not so much for armor penetration, and you're either going to need some kind of hellgun or nonstandard ammo packs. Check out Hot-Shot Charge packs in the DH core, under "Ammo" - you have to reload after every shot no matter what the clip size is on your gun, but for that one shot you gain +1 Damage, +4 Pen, and Tearing. Can't be used with autofire, obviously, but put that in a long-las, take an aim action or two, and that'll kill things dead, fast, and cheap. (There's a big turn tax on it, obviously.) Or just point-blank full-auto with anything, that's usually pretty deadly.
If you're trying to kill lots of people, you'll need to be using full-auto fire, and your primary goals will be lots of BS (you need one success per hit, so if you want to hit four guys with one shot each, that's four successes you'll need) and full auto RoF (you can't take more shots than your RoF, obviously.) Full auto is not something that lasguns do lots of - they do semiauto, which is great for ammo conservation and terrible for racking up lots of hits - so short of a multi-laser, your options are (once again) a D'laku Hellgun (that's Skitarii standard-issue, so good luck getting your hands on one - easier to aspire to one of the hellguns in Rogue Trader core, which have AP 7 because that book came out after the 5th edition Imperial Guard codex) or a Minerva-Aegis Las Carbine (page 112 of IH - Scarce availability and a cost of 150 puts it a little bit above a long-las in terms of difficulty of obtaining, but it's a good all-around gun.)
Or, if you're willing to slum to pistol-class weapons, you could pick up a Fury-class laspistol, which is a personal favorite; the range is terrible (it's the same as a meltagun), but they're not TOO hard to find, and they have a full-auto rating.
Or you could just get a good autogun.