Well he mainly played there when he was called to the Dodgers last year. Interesting thing is he's only played 20 games at 1B at all levels as a pro, but i'm sure he's an option there too. I think the main thing is that the Cubs have options at 3B, 1B, and DH now with Morel, Busch, Mervis, and Wisdom, plus Madrigal as a defensive option at 3B. They can platoon these guys based on the matchups.
Hopefully Morel/Busch are able to hold down 3B, at least until Shaw is ready, and gotta think somebody between everyone mentioned is going to be dealt at some point when that happens, if not this offseason. 40-man is tight. Mastrobuoni might get put through waivers or dealt.