Best approach for building multiple rail isolated dc-dc converter - Multiple isolated DC from single DC source
My first knee-jerk reaction is to do this with flyback converters.  Normally for a few Watts, you can leverage the high volumes of power over ethernet transformers.  15 W might be a bit high for that.  Look around.

If you can tolerate some ripple, all you need is a free-running oscillator producing pulses to drive the gates of the low side FET switches.  Each isolated side drives a opto-isolator when the desired voltage is reached.  The output of the opto back on the non-isolated side forces that FET gate low despite the oscillator signal.

I've done supplies like that using the PWM output of a microcontroller, with the opto signal feeding the fault input of the PWM generator.  That takes no firmware overhead once the hardware is set up.

If you don't need any other logic, then just a free running oscillator at maybe 200 kHz is sufficient instead of a microcontroller.  Each FET needs its own mechanism to force the gate low independently when its opto signal is asserted.

If you keep the input voltage to below 30 V, then you can use "logic level" FETs that can be driven directly by digital logic.  For example, the IRLML6344 would do nicely.  It has only 29 m&Omega; Rdson with 4.5 V gate drive.