Sparkler, I am pretty new to this whole thing as well, but....it seems that I have a similar issue as you in that I have this "in love/newness/lust/nre" thing STRONG for our third. I have such a strong attachment with her, and LOVE the idea of a new caring person in my life. She, C, loves me too in a purely 'friendship' way, although her actions sometimes say 'more'. I try NOT to read into that and just really enjoy HER compartmentalizing of it, ie, our hang time together, and then our play time together.
I have told both my hubby and her that this friendship love (one step up though) coupled by the lust she exhibits in play time are enough to keep me satisfied.
The difficulty for me has been when my hubby and her are alone, or having private moments. I haven't really got my finger on the 'why' of that yet, but assume it is a bit of 'rejection/non inclusive/control' thing going on.
When I am within arms length of them, I have absolutely no issues with anything.
I have had trouble compartmentalizing as well...but it seems to be getting better. Early on when we would have drinks on the deck or whatever, I just wanted to touch her. Deny deny deny....and eventually it becomes normal.
Good luck. It's all about finding a type of love that you CAN share. IMHO.