Class 2: Dems gain CO, NC, IA, ME (if Collins retires), MT, AK, GA, KY, KS, and MS (with Hood). D+10.
Class 3: Dems gain PA, WI, MO, NC, IA, AZ, OH, FL, KS, GA, LA, and IN. D+12.
Even I think that's a bit generous to Democrats. While I think they have a barely fighting chance of winning in KY, Mississippi and Kansas are way too out-of-reach for them. Kansas Class 2 was only competitive in 2014 because the Democrat left the race and Roberts faced an Independent challenger who refused to say which party he would even caucus with. Kansas Class 3 was never competitive and I just can't see how they could win there, no matter how much damage Trump/the Kansas GOP causes. I think they could win IA Class 3, but only if Grassley retires. OH is just a little bit out there(at least against Portman), and Louisiana and Kansas are way out of reach. Indiana was only competitive in 2016 because of former Senator Evan Bayh entering the race, and even then he ended up losing pretty handily. I have my doubts about IN.