Sewer odor from basement floor drain
Just cover or seal it since it's not being used for anything. If it has a screw-down grate you can take that up &amp; punch the screws through even just a plastic lid or its portion &amp; screw the old cover back down for a solid seal...coffee container lid, peanut can lid, etc.

The Home Improvement stores also have plumbing plugs for most sizes of pipe, which are more permanent &amp; better sealing. They expand as you turn the wing-nut to lock into the pipe fairly well. Plug all 3 drains so the house can't fill with either methane or just stink at any time.