Checking Third mirror alignment

First, put masking tape over the tip of your laser nozzle

Then re-attach the nozzle and pulse to make a hole through the tape.

Finally without removing the tape look through the back of the nozzle to confirm that the beam is travelling cleanly through the nozzle opening and you aren't losing any power in the nozzle.

If your beam is traveling through and you can see masking tape all the way around the beam hole then you aren't losing power at the nozzle. 

Otherwise, you'll have to adjust the 3rd mirror and shift the beam such that it is traveling through cleanly.