Garnacho didn't really get going this season until December when he got moved to the right. Before that he was basically at the same level as Rashford this season. Well, in possession anyway, obviously he was working harder defensively so his overall game was a bit better. But it's only been since December that he's been fairly consistently good.
Mainoo didn't play until December, and while there's no doubt he's impressive in moments he does drift in and out of games a lot and struggles to consistently impact them.
Dalot hasn't been amazing or anything, but he probably has been the most consistently solid throughout the season. He's had the odd game here and there that he's struggled in (the recent Chelsea and Brentford games for example), but overall he's been quite good.