Alfonse

Unfortunately that's rather tough since 3*s have low stats all around. You can try but it's rather difficult and you'll need to rely on your other heroes to help chip down enemies for Alfonse to get the kill.

Well I just did the missions with a 4 star brave sword Alfonse and it was kinda annoying XD. If anything give him a brave sword and keep resetting the stratum until you get many green units.

My Alfonse is also 3 star and I was able to do it. I gave him heavy team support through Hone Atk and buffs from B!Lucina. As others have said, try refreshing the Training Tower until you get lots of healers or green enemies.

Honestly we get so mamy feathers these days. The Askr trio will always be used in this game so 60k feathers to 5* them all is a great investment long term.

Well, the stats of a 3 Star unit is rather unimpressive, but if he's at least 4 Stars he'll manage.

Though, I had already made my own Alfonse 5 Stars a few months back and he runs a Brave Sword build. So I definitely think that, if possible, you should get the Askrian Trio to 5 Stars to make use of all of their stats and some other significant uses.