LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.
If the third LED is not visible after the system is turned on:
Verify whether the boot chime is present and fans are running when turned on (reset the SMC and PRAM and verify backup battery voltage for proper startup).
If the POST boot chime is not heard, go to the No Startup symptom flow.
If the POST boot chime is heard, go to the No Video symptom flow.
So, now we need to know if you get the Post chime. You also want to make sure you check your board for any oviously damaged components etc.
Here are the trouble shooting guidelines which will work for your computer No_Power-Diagnostic.pdf