I have a 5.7 that has the centralized sequential injector system and something is wrong. When I turn the key to start, the system "floods" #3 & #5 with fuel, so much that I had to drain the oil(it dumped over two qts of gas into the pan). What would be the problem and what should I attempt first to fix the problem?

 

Any codes??..You say 3&5,which is on the drivers' side...Most of those pressure regs leak eventually,but on the other side

 

Already replaced the pressure regulator prior to the problem and drove over 100 miles . No codes pulled yet.

 

Then you either have..
Faulty injectors...
A grounded wire somewhere causing the flood
A bad pcm

I have no clue of your technical ability/tools...but this will be a crapshoot without some real diagnosis....