Can Bunker's Grenade Launcher be used on the same target 3 times?
No, you must select different targets for each set of damage.

In the Sentinels of the Multiverse app developed by Handelabra Studio in cooperation with Greater Than Games, playing Grenade Launcher asks you to select a target for each instance of dealing damage and does not give you the option for selecting the same target(s).

This matches the usual definition of "another". From dictionary.reference.com:


  Another (adj):
  1.
  being one more or more of the same; further; additional:
  another piece of cake.
  2.
  different; distinct; of a different period, place, or kind:
  at another time; another man.
  3.
  very similar to; of the same kind or category as:
  What we need today is another Thomas Jefferson.  


In general, Sentinels of the Multiverse cards are very literal in their instructions: if it were intended to have an OR, it would have an OR.