VivianRixia

It really depends on what they are into.
Ideally it will be something short and self contained. I’ll start with the gold standard for recommendations: “Cowboy Bebop”

If they want something light-hearted and comedic, “Spy X Family” is a good one
“Vinland Saga” for something heavier and more serious

For fantasy fans there’s “Frieren” and “Delicious in Dungeon”, even if both are part 1 of a larger story
if they want fantasy aimed at someone younger, then “Little Witch Academia” is a good pick

If they want a combo of sports and romance, I thought “Yuri on Ice” was top tier.

With those out of the way, since you only said 3, I’ll pick these 3 to give a wide variety of what anime has to offer:
Cowboy Bebop, Delicious in Dungeon, Spy X Family

I think its even better as the new arise power gives us some personalities to follow instead just just our solo protagonist leveling solo.

Slime movies are kinda mid, except for Diablo who is always gold. Here’s to more Diablo content.