2001 Chevrolet Monte Carlo Ls Chevy 3 4engine Just Replace Old Starter Brand New

I have a LS Chevy Monte Carlo 3.4Engine
I just replace my old starter with a brand new one
And I just replace the battery and my car isn't
Cranking over and when my mechanic try s
2 jump from the starter it's cranks than turn over
For 3 seconds and it lose power and wen I turn
It from switch its blank like no power and it just
Stop cranking over wen I went 2 gas station
My mechanic first said it could be the starter
Is defective then mechanic said eginition switch
I change that stil doin same thing now they say it
Might b COmputer control module I don't want
2 waist no more money