I never played the original Just Cause though I can extrapolate what sort of a game it was and whether I would have enjoyed it or not from JC2 and JC3. I think I would have. Just Cause 2 was a really fun game in the vein of Saint's Row (but more P.C.) and at the time, the grappling hook gameplay innovation was something really special and added a tonne to the way gameplay felt. Just Cause 3 is another iteration on the original concepts of Just Cause - a big open game world and a new gameplay mechanic added to the mix.
I'll tell you up front that I enjoyed the game but felt that it overstayed its welcome more so than JC2 did.
As usual: Thar be spoilers!