I agree with you about Alice, she needs a LOT more attention.
There are times when the MC acknowledges that he's been neglecting her and that she deserves more attention, and then he seems to go out of his way to hang out with any other girl except her.
I get the feeling that the MC sees her as more of a sister than he does with the other two, possibly because of their shared history with Martin. I think that has pushed him into a more traditional protective big brother role rather than the boyfriend role with the other two.
He seriously needs to make time for her, not just one off special events like a date followed by a night in a hotel. If she's that horny and time is limited then even a quickie in the bathroom would do.
That's the problem with harems, it's difficult to give each woman the attention she deserves.