Well, my favorite unit, for the moment, is Yukiho-Makoto-Chihaya. I'll start by saying that all three idols are awesome. ^_^ Now for specific parts of the relationship...
Yukiho and Makoto are already the best of friends. At first, I didn't want to put them in a group together, mostly because of all the shipping the two get. So, in my world, there is no pairing, and thus I can happily live with them simply being BFF's.
Yukiho and Chihaya... I'm not sure. From what I have seen, Chihaya would be immensely annoyed with Yukiho calling herself flat-chested. However, I can see the two getting along well; once Yukiho gets to know Chihaya well enough, the latter would lower her guard and become more social with her, and the cycle gets going. However, this would be slow to start without a catalyst with a strong personality. (coughMakotocough)
Chihaya and Makoto is interesting. Chihaya is cool and serious, while Makoto is intense and a bit tomboyish. I actually think they might get along fairly well. I can't see Makoto giving up on becomming friends, myself, especially if they're in the same unit.
Now, as a group, I can see a bit of tension around Chihaya at first; she's seperate from Yukiho and Makoto's BFF-ness, and not exactly the quickest at making friends. However, I can see her getting accepted as one of their friends with ease, and the three would have a very easy-going vibe to them.