Now that these are on sale, I'm thinking about getting them. Is deadly games considered part 3? or an ad-on of part 2? Which ones would say are worth getting. I'm thinking of just getting 1 & 2.

1st there was JA 1, followed by Deadly Games, an offspring which shares basically the same functions/design,
imo perhaps both worth to buy for a short visit but nothing i would want to play through many times,
in opposition to JA 2 respectively the JA2 1.13 overhaul, probably the game i would want to have on a lone island.
JA2 1.13 adds a ton of switchable features to the vanilla game and is the base for several high quality mods,
each on its own has more to offer for free than what you would expect from the costly so called dlc content for games of today
Finally there is JA 2.5 aka Unfinished Business, an offspring of JA2, also worth to buy if you like the latter.