each motor would need to meet the appropriate requirements in Art 430 such as overload and ground-fault/short circuit protection.
As far as feeding them, if the feeder is sized per 430.62 and the tap rules of 240.21 are observed you can tap from a feeder.
Upfront the basic math of 3 x 20 is greater than 50 would indicate a problem.
For a more detailed answer, more detailed info is needed.