Lately I've been doing a really quick leg workout, Compounds sets of overhead BB squats with unilateral leg pressing. Really fries the quads and the hams.
Child's play. Try this: Warm up with any way you choose, then do 2 x 100 reps for leg extensions with very light weight to warm up your knees. After that, load leg press machine with 90% of you ORM, and leg extension machine also with 90% your ORM. Now you start with the leg press, and do as many reps than you possibly can, all the way to the failure. Then, without any rest, you run to the leg extension machine and repeat that doing so many reps you can, all the way to the failure. If you can use some spotters, you can do leg presses as a drop set to get the maximum result. One set should be enough, if it isn't, you haven't give all you got in the first set and should start with even heavier weight or more reps. That really is 3 minute legs