All PC PSUs with 3 or more outputs use forward converters because they are better for many reasons; lower impedance, lower cost, more stable over a wider range of loads, and input voltage and only need 1 feedback from the main voltage with tight cross load regulation (low error from high mutual coupling.) as I said look at any design of these type and understand why they can sell for 0.15 cents per watt. You also want to consider active PFC.
but if you don't have good (detailed) design specs, don't expect good results
you will run into problems with light loads on flyback overshoot and sub-switching resonance as well as EMI on max load, if this is your first rodeo.