Hi there, this support forum is for rubygems platform issues...not usually for dealing with specific gems. We don't want to support all ~15k gems through here, and usually gem authors do a fine job of it.
I would look through that gem's readme to figure out where their support channel is. For starters...it sounds like you're on Windows, and many gems have C extensions (bson_ext), and you don't have a C compiler available. Beyond that I would bug the maintainers of either that gem or the mongo3 gem to see what's up.