input 36-75V , output 12V => buck converter
and yes the 1:3 step up @ 20Amps output is the problem you can't do that with single inductor and low ripple regardless of the frequency.
I know some of us have thick skulls, but explain how its spec'd for 1500 volts isolation between the input and the output.
anyhow, as far as the 1:3 step up, Sepic and Cuk converters have serious issues running near zero volts output due to control issues, though it would be possible to use either of those topologies if no isolation is required, however, the issue is synchronous rectification. i don't know of any off the shelf control chips that will let you do that.
I would rather stick with a forward converter, as it offers operation across that zero bound better and can be controlled as a buck converter, or a boost converter if you want to use a flyback topology.
alternatively, you can use a boost buck topology, rig up a 3 phase boost converter and stick a buck converter on the other end of it. such a system would require about as much silicon as a flyback, but what the hell, it gets the job done