I'd like to know the difference in the following points between the B3 13m and B5 12/14m to find out which one should be closer to the B3 13m in term of low end and turning speed.

So does the B5 12m could replace a B3 13m? Or the 14m is needed to get the low end?

Thanks,
Seb

That's a tricky call. You'll have to take the 14 to get the low end of your 13 and you'll have to take the 12 to get the turning speed of your 13.
Ah and not to forget the relaunch of the b5 is no where near as close to your b3.
IMO the b3 was the more complete all around kite they've made.