Hi Bolivar, having had the pleasure of up close experience with all the boats mentioned, I am going to be politically incorrect and say go with the blade. the 20kg difference between the blade and the other two boats is a difference of night and day. of course if you have the pleasure of test sailing all of them, be my guest, but the rig on the blade alone, assuming it has a super wing mast, is worth buying the boat for. I'll probably get flamed for this, but really the rig alone is that much better than the other two boats.
to answer your questions, all three of them have different techniques of getting to speed, so i would say all three are equally difficult or easy, depending on the way you look at it.
I would say the blade would have the most spohisticated mast design, plus if you get the fat head main, it opens up a whole new performance level.
The intern 17 is a older hull design, the blade is the newest. I would say the FX one is a hull design that aspires to be the blade but doesn't quite hit it. With the "planing" potential of the blade, I would imgine that it would be a faster hull design. But don't get me wrong though, all these hull designs are fast and even the blade, which i feel is the fastest hull design is not going to make up for it if you start late, tack the boat badly, bang the wrong shifts, etc, etc. It still all comes down to the sailor for the most part. Hope this helps