I understand. On that third set, if you still feel like you have plenty of gas in the tank, I don't think it hurts to go ahead and pump out a few more reps, if you can keep form solid. In the Bigger, Faster, Stronger program (a basic compound, 3x5) they say to do as many as you can on the third set, not over 10.
One coach, John Christy, would recommend occasionally pushing the last set until 1-2 reps short of failure. So for example you'd do 2 x 5, and then the 3rd set do maybe 6, 7, 8 reps if you could manage them. If not, you know you're really working the right weight for 3 x 5 because you can't do
more than 3 x 5. But if you get 2 x 5, 3 x 7, you know you're working closer to your 7-rep max and you can bump the weight up just a bit more than you would if you only got 3 x 5.
So you can use that last set to see if you can go up a little faster. Go up 10# instead of 5#, or 15# instead of 10#, whatever, because you know you've really got it in you.
Just another suggestion. I mean, I've got no issues with people working 3 x 5 or whatever, increasing the weight slowly, even if they're spending a few weeks below their "true" maximum. It's not like taking it slow and steady is costing you anything but time, and it gives the body plenty of time to adapt to all the work your making it do.
Hope that helps,