Who is making a retention holster for the M9A3?

Safariland. Same holster for any railed M9 holster

Ok so I can use a holster from them that fits a Vertec M9A1?

Any M9A1 or Vertec holster should fit.

Yes

I got a Safariland 6280 for a Beretta Vertec with M6 for my M9A3 that I have a TLR-1HL on. I ended up having to heat it where the front of the slide slims down near the barrel on both sides of the holster to get it to fit properly.

Out of the box the holster wouldn't let the pistol go in the last 1/4" or so and if you pressed down on the grip you could watch it push the pistol out of battery and the hammer was almost touching the SLS hood and it was hard to disengage it. Took me about 15 minutes and a bunch of test fits as I didn't want to go too far.

You may or may not run into that issue, just wanted to throw it out there.

I also got a concealment holster from IBXtactical for it (which I highly recommend btw, he does great work and was extremely accommodating in fixing my order do to a misunderstanding between us). If you get one from him and you want a full sweat guard make sure you put in the order notes to bring the guard up to the back of the slide near the hammer as by default for a Beretta even with a full sweat guard he ends them before the de-cocker.