From what I have been reading, most people are not very impressed with Ele1. The cast times are insanely long and the damage isn’t as good as you would expect. Electrocute, which should synergize well with Cryo, apparently has too long of a cast time so it is too difficult to use on frozen enemies for the extra damage bonus. Apparently Ele2 is alright, and Ele3 is even better though.
I do not think there is a single person who is impressed with Sorc (myself included). They are decent and have a really cool system, but the insane SP consumption is just too much of a hassle and takes all of the fun out of the class.
There is not a single Necromancer CBT2 review, so I have no idea how that class plays, but it certainly looks fun.
Having said all of that, I would personally go Ele1 -> Sorc1 -> Necro1. The classes are most likely going to change in the future and this way you will get a better feel for how each class works, and you will have a better understanding of which class you want to play in OBT. If you just want the most OP class of your choices, go Ele3 though. You will have a harder time levelling with the other 2 choices (Sorc pretty much gives you 0 useful skills to use unless you want to spam SP pots 24/7). .