Board Thread:Game Discussion/@comment-67.187.25.76-20130508083413/@comment-8284783-20130508152036

As Arcturus suggested, events are probably the best way to get new R3s.

The bigger the bosses you help defeat, the more (and better) reward medals you get.

Last event I got several hundred silver and gold medals just by helping out against the bigger bosses (those with bounties over 2k start giving out good rewards).

I'd say you have around a 2% chance of pulling an R3 per silver medal, and around 5% for gold medals. I don't know about R4s since I never pulled any : (

Outside of events, running missions to get the combat cache or a boss' weapon would be the best/easiest/most assured way of getting R3s, but you are a lot more limited in which ones you can get (whereas, with medals, you could pull any rare).