What's the order to watch as I have the seasons of:
Blood and Sand
Vengeance &
Gods of Arena
Blood and Sand (season 1) -> Gods of the Arena (prequel side-story) -> Vengeance (season 2)
Gods of the Arena was created because
Andy Whitfield, the original actor that played Spartacus during season 1, came down with non-Hodgkin lymphoma, and the prequel would give him a chance to recover. Unfortunately, he died, and they ended up replacing him to film season 2 and the now-airing season 3 (War of the Damned). To be clear, the prequel can actually be watched prior to Blood and Sand, but I think it's more interesting to see the beginnings of some of the gladiators that you met during season 1... Crixus being the big one.