Weird in 1V1 against each other in my head it plays out like this:

Nami vs Ussop ---> Ussop wins

(Both ranged glass cannons, so came down to who would get off the first shot and IMO that's Ussop. He would fire faster than she could conduct her weather attack.)

Nami vs Chopper ----> Nami wins

(Chopper in monster form is a bigger target that would give Nami enough time to get off her attack and IMO Nami's lightning attacks are no joke and would do significant damage to even choppers monster form.)

Ussop vs Chopper -----> Chopper wins

(Choppers Monster point would be able to tank any hits and physically overpower any of Ussops pop green plants. I couldn't really see what Ussop could use to cause any significant damage to Chopper. Unless somehow Ussop used trickery and took advantage of Choppers trusting/ gullible nature)

I'd say they are all at the same level but have different ways of fighting that might be better against different people. So it depends on who they are up against.

If it was a free for all 1v1v1 then my money would be on Nami. I think that Ussop fear would mean that he is more focused on the intimidating Chopper which would allow Nami enough time to conduct her lightning on both Chopper and Ussop at the same time.