Are these good prices for these lenses?

$50 for a 50mm E Series F/1.8
$25 for OTOMAT SERIES-35 NIKON AI MOUNT 1:4 F=75-200MM MACRO LENS
$40 for NIKON AF 28-80mm D
These are all used lenses that I found on eBay.
Also what's the difference between AI and N/AI lenses? And will they work on my D3100?

I already answered this, but here goes.

N/AI is a code used by third-party manufacturers to state that the lens fits Nikon (Nikon AI).

AI lenses are made or modified by Nikon.

Series E lenses are AIS, designed for Nikon's manual-focus multimodes.

The Otomat is a piece of junk and I'd worry that there was something wrong with the 28-80mm at that price - the price for the 50mm seems fair.

All will fit your D3100, but none will autofocus, and you'll lose most (if not all) of your exposure modes with the Otomat and perhaps the series E.

For full functionality, you need lenses in Nikon AF-S mount.