greed island is ova2, is 8 episodes, and takes place immediately after ova1, and is a direct continuation of the storyline (albeit into a new story arc).
gi-final is ova3, is 14 episodes, and takes place immediately after greed island, and is a direct continuation of it.

So if you're leechin' and watchin' ... find all the regular season eps, then find ova1 somewhere (it should just be labelled "ova" or something like that, check your local dc hub), then find greed island, then go to greed island final. Anything else and you'll be watching out of order, and confused by the gigantic leaps in storyline.