Couple points:
A person with a knife is extremely dangerous. A person can run 20 feet and stab you with a knife before most people can unholster the gun, nevertheless get a shot off.
Second, a taser has an inherent problem: there are not one but two prongs that come out, with one coming out at [I believe] a 10 degree angle downward. This is what makes the electrical connection that shocks you. With a guy holding a knife and coming at you, you simply can't endanger your own safety by getting close enough to him to accurately use the taser and ensure both prongs will hook on.
I do think three shots were arguably excessive, as one probably would've stopped the threat. In CCW classes we're taught to shoot three times as we unholster and extend out, basically going for lower midsection, center of mass, and upper chest or head. But for a cop whose gun is already unholstered and aimed, I think it's likely one well placed shot would've stopped the threat.
Still, personally, I'd rather a person like this be in the ground, anyway. No reason to enable these types of dangerous people just because they're "mentally ill."
leg shot?