All three are currently made by Footsore Miniatures (who I think are maybe being renamed Grey for Now Games or something?).
I don't have any experience playing any of them, but Test of Honor and Mortal Gods appear to at least use similar force structure, with a focus on a small number of heroic individuals and supporting goons based in groups of 3.
Gangs of Rome seems like smaller scale skirmishes, with only a handful of models per side where each one is a more developed character, and as far as I can tell is "street fighting" between thugs, rather than soldiers fighting each other like in ToH/MG.
Hope that helps!