I love how everyone always thinks people are asking, "Should I be in this position because I didn't scout?" I'll try to answer the question as asked:
If they have six Scarabs, their bases are undefended. So, while I agree that it shouldn't have gotten that far, assuming it has, I'd avoid fighting the scarabs, let them attack your bases.
The bases will die but a few things will help that take a while including: scarabs are not particularly fast, pathfinding in the game has some issues (and I can almost guarantee that six scarabs will spend some time stumbling over each other), and you can slow them down with Ander's ice or shield gens (destroy a building or two if you need to, eventually you'll want to sell everything on the structure for the cash before each base dies).
While you're buying time, use all of your combined forces to destroy their unprotected bases and build on the ruins so that you stay in the game when your old bases die. Ultimately, they'll have to split up their forces, which you could probably handle (if you took out 2.5 when there were six, I'm guessing you could've killed them all if there were only 3, maybe even 4) or they will be racing you to destroy all bases. Still, if you're careful to keep picking up the stray bases, I'd like your chances.
Depending on what sides you were, there are other possibilities. But given the situation, I'd say going heavy offense is the right move, even if it seems counterintuitive.