What lens to add to 18-55mm and 55-200mm setup on a Nikon D3100?

Should I swap the 55-200mm for a 55-300mm or 70-300mm?
Also what's the best landscape lens?

Your 18-55mm is perfectly adequate for landscapes at the short end.

I suggest that you concentrate on learning how to get the best out of the lenses you have (capable of handling most situations you're likely to encounter - if you know how to use them) rather than throwing away money on kit you don't know how to use.

There's no significant advantage to upgrading to a longer lens right now.

For landscape work, you could benefit from getting a wider lens, but it's not totally necessary. Something like the Sigma 10-20mm f4-5.6 would be a good budget option, if you want a spendier option the Tokina 11-16mm f2.8 performs better. The 55-200 is also useful in landcape.

If you don't have a good tripod, then that is more important than getting a lens, this is essential for serious landscape work. I suggest spending at least £100 on something from Giottos, Gitzo, Manfrotto or Slik.

Yes you can swap the 55-200mm to a 70-300mm. You won't miss the gap. For landscapes, if 18mm is not enough for you get a shorter focal length lens, lower than 18mm. One more lens to consider is the 50mm or 35mm f/1.8 or f/1.4 for shooting in low light and for speed.

I'd say a 35mm f1.8 DX or one of those $169 500mm ebay telephotos. I have one and they don't have super optics, but they are a lot of fun. Adds a whole new dimension to photography.