No. . .
Bristlenose plecos actually do need 20 gallons of water, it's not about the amount of water is about the footprint of the tank, they need 24" x 12" base footprint to thrive, which is a standard 20 gallon (long or high either one). Even though they only grow to around 5" or so they are still pretty active and need that big of a footprint in order to thrive. It's like putting a puppy in a box and expecting to be content with the space that s/he has, obviously the puppy wants to explore more and have more space to explore.
I highly suggest you return the pleco, along with getting more girls, three isn't enough. Bump the numbers up, with either 1 or two girls, the more girls the more the aggression gets spread out. Along with that I also suggest bumping the numbers of shrimps. :) They like to be in a colony of five or more, they're social beings too.