Soul Blazer's a bit longer than IoG, but I have to stress that that's if you know what you're doing in each. IoG is more complicated a game, so if you get lost or hung up on a puzzle, you'll be there for a longer time. Soul Blazer barely has any puzzles to get hung up on in the first place.
I dunno, I quite enjoyed all of them, to be honest. I thought Soul Blazer was a more...unique style than the other two. Not to mention, it might be a bit off putting when you're playing games with graphics like Terranigma and IoG, and then moving right over to Soul Blazer. I would have said start with SB and then go from there to IoG, and then save Terranigma for last.
??? I thought IoG was the longest of the 3. Soul Blazer is a cakewalk and can be finished in a single day, easily. IoG-- it took me weeks or months, playing off and on. But I feel like I put 20-30 hours into the whole thing.
I finished IoG two weeks ago, really good game but it doesn't compare to Terranigma. The game was really short, I guess it took me around 10-12 hours to beat it. It's definitly shorter than Terranigma.