Yeah I have a deep love for Vanquishers so I'm dying to run one with Pask, a lascannon and camo netting, but it's so expensive. About the same cost as a landraider! In a normal game, I can't see including one (I'll still make it and keep it around) but maybe they could really shine in Apoc where people tend to roll out the heavy armor. Otherwise I'd worry about running out of targets. Without tanks to shoot, it's got no other purpose. Also I agree with Kevin, I'd never even try one without Pask. Guard BS never fails at missing when you need it.
Something I've been toying with is trying an Executioner with Pask. 4 Twin linked Missile strength shots against vehicles and MCs could work out pretty well. And when all the tanks are gone he can still mow down troops.
As for taking 3 of them, I'd advise against it. Barebones with lascannons that's a full 1/6 of your army given to 3 guns with crumby BS and no real secondary roll.