Who to upgrade to 5*

Roy is the best one of those three. I wouldn't recommend spending feathers on ranged units since the way the game works they are useless in arena.

First, Chrom is a no go.

I agree that Roy is incredibly strong and if you find yourself having problems with green units, he's a good choice due to triangle adept. The downside to him, however, is that he is essentially irrelevant against ANY blue unit and will get doubled against the current meta reds like Lucina or Lyn.

Nino is tricky. If you had a 5* that could bolster her stats (Eirika is by far the best for this) then she would HANDS DOWN be the best to upgrade as her bonuses add to her damage and allows her to double a good majority of heroes with a 50+ ATK stat. This allows her to take out even the likes of Hector in a single turn. Combine that with a dancer and you're looking at a crazy combo.

All things considered, with Nowi being able to give your Nino stat buffs, and really the only other viable Green users being Julia and Hector, Nino would be the better choice as she can more than likely outtrade both of them and gives you a huge advantage against meta blues like Robin, Nowi, Reinhardt, etc.