What lens to get for Nikon D3100?

I have the kit lens a 55-300 and 50mm f1.8 i'm looking for another good portrait lens my budget is $800 max

35mm f/1.8?

One possibility could be to replace the kit lens with a 16-85mm AF-S VR. Another is to get a 60mm f2.8 AF-S macro.
Actually the 55-300mm could be a good portrait lens. Some people gush about the 50mm f1.8 being a great portrait less. (Is it?)
However, if you don't already have them, a good tripod and flash should be on the shopping list ahead of another lens.

What do you need another portrait lens for? Unless you specifically know something that your current lenses are not doing for you, you should be pretty much well equipped in this regard already.

You'd pay a lot for very little gain if you buy yet another portrait lens.

If you insist on what i think would be wasting money, consider a 60mm/2.8 macro lens, a 85mm/1.8 prime, or the Sigma 50-150mm/2.8

Both the kit lens and the 50mm f1.8 ARE portrait lenses. Why you'd want another one is beyond me.