I have not had the chance to fire a wood burner, so can't comment on that.
I would say that firing a coal burner requires the most skill in knowing when and where to put the coal, and getting each scoop to the right place at the right time. Proper firing makes the job physically easier. It's not simply a brute force operation of shoveling X number of tons into the firebox.
However, to me firing an oil burner is more difficult in that, to do it well, requires constant attention. You have to watch the steam, water, stack, and the engineer. Every adjustment the engineer makes requires a change in the fire to compensate. No gazing out the window for a few minutes and watching the scenery go by like you can get away with on a coal burner.
But that's just me.
Russ