Why is the series "Trigun" titled as such?
It's not related to the number of barrels of a gun, but the number of guns Vash has.

1st: Is the silver 45 named 45 LONG COLT AGL FACTORY. This is the one we all see him carry.

2nd:


   the hidden gun inside his arm that we see in Episode 12 This hidden gun also gets upgraded into a machine gun in episode 19.


3rd:


   the transformation of his arm into the Angel Arm that we see in Episode 16