Yes, yes you are. And this is exactly why people don't like Banthas. With the right cards and timing, you can absolutely devastate your opponent with a massive amount of unmitigated damage, and they are all but powerless to stop you, aside from keeping their distance and focus firing until the bantha is dead.
You are getting two Stampedes (once during the activation and once during Jundland Terror) and three Tramples (two during the activation, and one more at the end of the round from Jundland Terror.
Single Purpose says "up to twice during the activation" and it applies to ANY special action, you get to choose which one when you decide to use it.
Jundland Terror happens outside of the activation, so it does not restrict another use of Stampede (since the rule on Massive movement is "during its activation") or Trample (Jundland Terror allows the Bantha to make another attack or ╔►, outside activation).
Edited by Fizz