Garnacho is 20 years old with 55 PL matches played in the previous two seasons. 87 in all competitions.
He should have a season now IMO where we don't overplay him. It happened to due us being over reliant on him, but that's not the case anymore IMO. He's a great option to start a lot of games, but also to come off the bench in a lot of them.
Rashford is turning 27 and barring injuries, he no problems playing 50 games per seasons at this point in his career.
Amad is 22 and hasn't had a lot of first team football in recent seasons. He's at a point where he needs a season where he starts most of the available games. Garnacho is at a point IMO where for his own sake, he needs a season where he plays a bit less than in the previous one.
If they're all in good form, Garnacho is arguably the worst player of the 3 as well, and I don't say this as an insult to Garnacho. He's a very good player, already experienced for his age and seems to have a good mentality and understanding of football at a young age already. But I don't want us to ruin him by basically playing 40-50 games every season from 18 years old. Could easily lead to physical decline by the time he's 28-30.