Thank you guys so much for the advice! This little adventure could have been phrased better than "first date" but that was what went through my head at the time. R and I have never really had face time, and she has been working at her own pace to (again for lack of a better term) accept me as his gf. The last relationship K had went south fast, nearly breaking them apart as well as his own deep friendships. Therefore, I understand her hesitation. She was the one who brought up this idea for dinner, which to me is a huge step for her. No way Im going to denounce her as "Queen of the Castle"! Im also probably not going to ask to be alone with K that night either. This is for the three of us.
As for how long Ive known K, how long Ive been with him and my willingness to be with them and help them in any way possible, I don't really think it matters. I want to be with him as much as he wants to be with me and by the Gods graces we are lucky to be involved in poly, same as everyone else here. It is my duty to support them in whatever way I can and those I deem to be moral so that their relationship will stay strong enough to allow my presence.
Funny part is, there isnt a time or day set yet, so Im just doing my preliminary worrying now so that Im not a nervous wreck the day of. ^_^