Hm, sounds like what happened in my/our case. The story you have asked for, is quite long by now, you can read about it in my blog (see signature). The first pages are about how we got things started.
There is lots of advice, but I guess the most important one for this situation: Do the guys know each other? Get along with each other? Aside from you and R working together in a shared living situation it is mandatory that John and R work in everyday situations and challanges as well. They should tend to their (platonic) relationship till you start living together and maybe a short trial setup would help work some things out and detect smaller or greater pitfalls. (shorter visit/vacation, something like that)
That's the way we did it and it worked out great
Edit: Ok, you may have to skip all the emotional stuff in the beginning, if you want to read my stuff. I/we were new to poly and didn't know our way around back then in regard to the general possibility of polyamory itself ^.^
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.