I am following '5 days splits' of Scott for 2 months and get really really good result. But i have big confuse about am i do 3 burn in all 3 sets or only in the first one ? because if i only do 1 burn set, my body didn't get fatigued enough though i had really lifted max amount that i can. Since this week i tried do 3 burn in all 3 sets and i fell much better. So should I do 3 burnsets or should I add more exercise for a muscle?

// One more, my leg is really weak compare to other parts of my body, and i often feel exhaust just after my squad ( first exercise ) though I only do 70lb ( including barbell ) max and it makes  me have really hard time to finish the rest and i always have to pass through the front squad. How could i improve my resistant in my leg? should I do cardio because my leg usually be fatigued in 3 or 4 days after Leg day :'( 

Every set should consist of all three burn sets (some have only 2 so make sure you pay attention to the write-up for each workout)

Honestly, you just need to work with whatever weight you can handle and push through. In time you will get stronger, give it a few weeks!