Would it also be correct to say "dans les mêmes vêtements" or is avec mandatory here?
No, "dans" is impossible.
It would describe a very bizarre situation where they would dress underneath or among someone else's clothes.
There could be two situations. They could be Wearing three same exact clothes like the other. Or using different clothes but from the same wardrobe. How do you differentiate between these two.
fun fact: max and leo are twins
Do the French use ampersand "&" for "and" ? I had this correct except wrote "Max & Léo ", and so got it wrong.
No, the French don't unless it's a logo or sign of some sort. Duolingo's algorithm doesn't use the ampersand either.
Sounds like a circus act, or a very big suit.