Is it possible today, or would it be possible in the future to trigger growth of limbs by forced Mutation?
Yes it is. Scientists are already able to trigger growth of ectopic eyes on fruit flies, therefore it is "just" a matter of introducing in the genome the triggering of the right genes at the right time for growing a third arm.
how many generations would it take to come to the point, where the third limb is as efficient as the other two?
That's not how evolution works. Bilateral symmetry (having two arms, two legs...) is part of the body plan of all vertebrates and dates back to the Cambrian explosion. If it has remained until today, it means it is pretty effective when compared to the alternatives.
As you state, polymelia is considered a defect because it is a deviation from the bilateral symmetry plan, and the only way to be transmitted to the offspring and be selected by natural selection would be that it gives a clear competitive advantage with respect to non mutated organisms.
Yes, nowadays a 3 armed man can be invited to talk shows and become a celebrity, maybe even find some willing mate for reproduction purposes. But if I look back at all the "freaks" who have reached their 15 minutes celebrities in the past, I don't see any of them having started a genealogy carrying on the same features which made them famous.
If you want your mutants to be successful, first find a clear case when they have a competitive advantage with respect to non mutants, then natural selection can kick in.