While I'm rather anti full-cream, what can I say, I'm biased, I do think if you're really struggling I would consider the frequency of your workouts. If you're doing heavy legs 3 days a week and struggling, my absolute first thought would be if I was going into it with my body ready. Proper nutrition, especially carbs, is certainly an important factor, but if you're body just can't handle it you may want to slow it down to 2 a week at least for a while and see how you feel.
So I'd try a few things independently. Up your carbs, obviously clean, and see how your energy levels are, especially with your pre-workout meal. I like getting some protein in for sure, but I primarily stack clean carbs and some swigs of my quick digesting PWO shake in the 30 minutes before I lift. See if this makes the difference, if it does, boom problem solved as long as you're still making progress.
If it doesn't, try altering your weight loads. If you're just not able to do manage 5x5 of the weights you're selecting, drop a bit and get your muscles filled out at that level before progressing. If you feel like it, try some stop pause sets (I forget what we call them around here), but finish what you can, wait 10-15 seconds, and finish the set. This is a great way to push those extra reps your body just can't do straight out, with more intensity than just working up to that number of reps. However this is a shocking method for sure, so I'd leave this for after you make sure you're energy levels are okay when it comes to recovery.
Which speaking of, if neither of the above are really giving you adequate energy and gains at the same time, drop your workouts to 2x a week, at least for legs, some 2-3 days rest between leg workouts. If your other lifts are doing fine at 3x, keep them up, just drop legs. Maybe on the day you were doing legs, do some active recovery with yoga, cardio, or even just some really light sets to speed recovery and keep them active.
Try each of those, independently. If any given one gives you energy and results at the same time, you're golden. If not, keep working down the list. In the end if you're still dissatisfied, try mixing it up. Take a few days to a week off, and hit it fresh, try some new shocking methods like drop sets, or if you're feeling up to it even try low weight occluded sets (use caution of course!) and see how your body reacts.
Perfection in motion. BS CPT - NPTI NASM
"We must not forget that even in the most perverted and cruel human being, as long as he is human, a small grain of love and compassion exists that will make him, one day, a Buddha." -Dalai Lama