Just one of them will last you for several hours. 3 > 1 > 2 in terms of easiest to hardest, or do I've heard. You can't go wrong with starting with the first, though. --- You only need to know two things to understand KH: Everyone is Sora, Everything is Xehanort's fault. http://backloggery.com/Lord_Frood
Have to agree that just one of them will last quite a while. Just recently got into the series, and I've already clocked over 57 hours on the first game. --- Stop reading my sig, you are wasting time doing so.
If you want to get into it Etrian Odessey 3 is a good place to start. It has the most options open to you. It seems easier over all, for various reasons. Finally this is most likely the easiest one to find, partly because being an Atlus game it needed to become Shin Megami Tensei popular to reach print run levels, to find it. EO1 didn't have that option and EO2 seemed to not quite reach that level.
Anyway if you can find EO1 for reasonable price grab it. You can start here perfectly well if you want, I mean it's were allot of started, and how it became a series in the first place. EO2 is only the hardcore, I need every game in the series type, and best played after EO1.
If you know you'll love the series for sure I'd recommend you start with EOI just because it's hard to go back and play 1 if you play the others first. I got EO2 when I wasn't done with 1 and I could not go back o<-< .....
Massive improvements from 1 to 2, and between 2 and 3 again! But I'd say 2 is a good place to start. 3 is also great I'll just say all the games are great (...) it just depends on how many of them you want to play
PS/ "it is hard to go back" as in, it's hard to get used to the frustrating controls and slow battles compared to the latest in the series (which you won't notice as much if you start with 1 in the first place)