now if you are thinking of coding your own engine it would be quite a task to make it available on 3 OS's
that being said there is a game engine called unity3d that is quite easy to work with and allows a lot of different scripting and programming languages,
ihave been fiddling around with it but i did have some trouble importing custom made models, lost interest in the meantime.
it is native for android and iphones as well as pc, mac and in browser 3d play using a specialized plugin (the battlestar galactica f2p mmo was made in this engine, it can look decent, not dx11 though, you might wanna try it as it can be played without downloading)
the whole process is very streamlined and you can try the engine out for free as well.
still no UNIX support though.
//edit//
i wonder if UNIX boxes can install the plugin in firefox, if so one could play it through the broowser quite easily, while till making a dedicated install version for pc and mac.(dont know might be fun to try out)
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//through chaos comes complexity//
the scent of the tamarillo is pungent and powerfull,
woe be to the nose who nears it.