No doubt Manchester United's forward looks much better this season compared to last season's mess, but the defensive line still lacks that composure in defending goals from their opponent.
Amorim should rather change the 3-4-3 formation and think of a better formation that will suit the team and give a try; As we can all see, the premier League is a very competitive league which he might not succeed with the formation this season. I could recall clearly how Chelsea struggled during the season with Conte introducing this same strategy, and it was a hard one for them to win matches and stay top as one of the big team in the league.
I don't see that as a given, as they've only played three games this season, so there's no real assessment to be made. I think Manchester United's overall performance is the same as last season, and the team's disorganization across all lines has led to numerous shortcomings, not just in defense.
The formation implemented by Amorim is a characteristic of each coach, and I'm sure Amorim knows the potential of the players in the squad well, so it won't be easy for him to change what's been done before; that would only risk worsening the team's performance.