I've said before I think there are underlying issues with Angelus and Dru. I think Angelus trained Dru to respond to him in that sort of way which means that Dru isn't actually making the conscious decision to sleep with Angelus but is just carrying out what she's been taught to do. Whilst there isn't "evidence" to prove this theory, I think there is at least enough stuff to suggest it.
I can see there being some of this in their relationship for sure, but I don't think that means Dru isn't happy with getting the attention of both Angel and Spike. She always seems to find their sniping at each other over her/Angel goading Spike pretty entertaining. It's hard to say how much of that is because of an acceptance that this is the way her relationship with Angel is expected to be and how much is just because she will just do as she pleases and she prefers to be with Angel too. Where the line falls is definitely murky, Dru is Angel's greatest creation after all.
As for the actual question of the thread, the answer is no. Spike doesn't like to share, but also he'd obviously previously suggested having a 2 girls and him threesome to Harmony before (possibly multiple times) because when she thinks Drusilla is a lookalike in Crush, she says to him "You'd better not be thinking what I think you're thinking. 'Cause my answer is the same as always. No threesomes unless it's (gestures to Spike) boy, (gestures to the air) boy, (gestures to herself) girl. Or Charlize Theron."
If Spike was open to having another male in a threesome, then he and Harmony would've been having threesomes.
Interesting point regarding his disinterest shown there in a m/m/f grouping, I'd never thought of that.
Obviously things are a bit different with Harmony because he doesn't care about her and isn't interested in having a loving relationship with her, so whether he'd even want another female to join him/Buffy is separately questionable.