I have never seen a situation where two “O” rings are used at the same time and face-to-face with another “O” ring. I have also never seen a situation where an additional gasket is used in between two apposing “O” rings.
One “O” ring should be sufficient to seal that fitting providing the fitting was intended to be sealed with an “O” ring. My regulator has an indent as if to use an “O” ring, but instead, uses a very hard fiber gasket intended for “one time use”. Break the seal, and toss the gasket.
Edit: Check to see if the two indents are exactly the same size. If they are, one ring should be sufficient. However, I might be mistaken, but it appears as if the two indents are different sizes. If that’s the case, you may have to replace the tail piece in your regulator. However, highly unlikely.