Should I say "3 half days" or "3 half-days" or "3 half-day"?

Should I say "3 half days" or "3 half-days" or "3 half-day"? I mean I want to refer to, for example, the a.m. of Monday, the p.m. of Wednesday, and the a.m. of Friday, together.


You should say:

three half-days