Dybala should play as a forward because he is a forward. Ideally he plays as part of a front two but almost no teams play with two strikers so that leaves him as the sole CF.
I am not sure it will be a perfect fit but Dybala's main strength by far is that his left foot has one of the best finishes in the world so he needs to be in a position to score otherwise why have him? His ability to drop deep and link up with midfield players as a false 9 is also underrated around here.
As far as the 3rd attacker goes (when paired with Ronaldo and Dybala) it has to be someone who with the discipline and work rate to often play as a midfielder. Douglas Costa is the best option for the RW in purely attacking ability but i'm not sure about his overall game. Both Cuadrado and Bernardeschi could do it but then neither of them has the ability in the final 3rd to create a goal scoring chance as often as Costa can.
As someone earlier today pointed out Cancelo probably would've been great for that role