In Mass Effect 3 for the PC, PS3, and Xbox 360, unlocking weapons to use in multiplayer games is based on a card system. You earn points to buy card packs (or you can use real money if you’d like), open the packs, and you get to use whatever you get from those packs when you play online.
Start with recruit packs, and mixing in a veteran pack from time to time in order to build your arsenal.
Spectre packs offer the best chance to get ultra-rare weapons, however they are also the most expensive. You’ll probably want to wait until you’re farming gold ranked missions before buying those regularly.
Unlockable Weapons from Card Packs
Here are the weapons you can unlock, and what rarity group they’re in.
Geth Pulse Rifle
Geth Pulse Shotgun
Graal Spike Thrower