In terms of overall difficulty, VI is the easiest of the bunch, the game was designed to be user-friendly compared to it's predecessors. With that said, it's the most complicated of the entries due to adding in game mechanics not usually seen in the genre and certain party members are designed for more "expert" play if you want to call it that. Still, I would say it's no different from things you've already dealt with in FFX for the most part.

IV is the simplest entry, I wouldn't necessarily call it the easiest entry, but it is the one that doesn't ask much from the player and is the most linear entry of the three. V is an odd middle ground. The gameplay falls into that "simple to learn, challenging to master" category and it tends to have a heavier focus on the player to explore and find it's many secrets to get the most out of it. It has the weakest story, but makes up for it by being one of the funniest entries.

I honestly feel VI is the best entry of the three for a "jumping in point" as IV is a bit too old school for modern taste and V lacks the gripping narrative that brought most of the fanbase into the franchise. With that said, I feel you should play them in order as it really shows you how the franchise has evolved.