The Resistance 3 beta is currently available to people who brought a new copy of SOCOM in the US and PlayStation Plus Subscribers in Europe however if you do not fall into one of these categories but want to play the beta then you are in luck.
Amazon are offering access to the beta to everyone who pre-orders the game with them, as well as access to the Air Fuel Grenade in single-player, the grenade releases a large cloud of flammable vapour when used.
“Pre-orderResistance 3and receive a beta access code as well as access to theAir Fuel Grenade, which releases a large cloud of flammable vapor, inResistance 3’s single player campaign. Air Fuel Grenade access code will be e-mailed within two business days after your order ships. Beta access code will be distributed in time for the August 15 access entry date.”
Insomniac Games recently released patch 2.01 for the beta which fixes some matchmaking issues.