I like characters with real depth in them, and no character with depth would agree to being a side chick for the MC.
Just playing devil's advocate here, it really depends on what the character is like.
For example, this is completely hypothetical and much smaller scale throuple instead of full blown harem empire, I can see a three-way relationship between MC, Erynn, and Alexis due to their complex relationship. It all started with MC liking Erynn when they were younger, but Alexis already semi-interested in MC from that time. Fast forward to the present time, Erynn confided to Alexis that Erynn likes MC, but Alexis fell in love with MC prior to realizing that the guy in the airplane was MC. So here's the dilemma, right? Who is the side chick between the two?
From Erynn's perspective, she felt that she should be first because MC liked her from back then, and she started liking him when he left. Plus, she already told Alexis about it. She felt entitled because MC liked her first.
From Alexis' perspective, MC was fair game because Erynn didn't like him back then, and though Erynn told her about liking MC, Alexis already fell for MC (and had sex with him too) without even knowing that it was MC. She felt entitled because she liked MC first all the way from back then. The problem is that she never told Erynn about it.
The complication then gets more complicated because Erynn and Alexis are best friends for such a long time. Do they sacrifice their friendship? Would one of them want to back off to preserve the friendship? How would Erynn react once she found out that Alexis had actually liked MC at the time when Erynn didn't even like MC? Would MC want to choose one and break the heart of the other?
When presented with such scenario, it isn't impossible for them to end up in a throuple (note that throuple exists IRL too though obviously extremely rare) because what they want is to be with MC, and not necessarily to prevent the other from being with MC.
But once again, just reiterating that it's a completely different scenario when compared to full blown harem empire where MC would bang his mom, mom's best friend, childhood crush, childhood's crush's best friend, childhood's crush's best friend's sister, childhood's crush's best friend's sister's best friend, the daughter of "dad"'s wife, the other daughter of "dad"'s new wife, etc.