It depends a lot on the regulation accuracy required and the load variability. Multi-winding flyback converters normally have one well-regulated winding and the others loosely regulated. You could for example design it for a well regulated 5 V output, a loosely regulated 50 V, and then use a small buck converter to generate the 3.3 V from the 5 V.
Another topology to consider, and which gives you all windings fully regulated, is a flyback outputting 50 V and a lower voltage, say 8 V, with the 50 V output being the regulated one. You then use buck converters for the low voltage supplies but only with a low voltage ratio which tends to allow higher efficiency. For the 5 V 10 A supply you will probably also want active rectification.