I’m starting to build a gas forge. I want to build a 3 burner. I don’t want to cut down the fire brick, so I have a internal dimension of 9x6 1/2x18 inches. I am also installing a Frosty T burner. Am I on the right path ? 

If you are not using an insulative firebrick you are on a very wrong path.

Morgan K26 bricks come in three different thicknesses. Also, they are easily cut to length. But, you will need to  use a finish coat on the inside surfaces. I would suggest Plistex; it is economical.

Not knowing what you plan to make, it's hard to say if you are on the right path or not.

Also a brick pile forge of about 1056 cubic inches inside and three burners is going to be a gas hog. So plan on a large hole in the wallet for propane.

Welcome aboard Jeremy, glad to have you. If you put your general location in the header you might be surprised how many members live within visiting distance. 

I have a 4 burner forge a little larger than that but happily I made it so I can partition sections. I've only fired all 4 burners a few times in all the years since I built it, a couple times to photograph it and twice to make room for 6 guys to use it. When I fire it up I almost never use more than ONE burner in about 350 cu/in. sometimes I'll light two if I'm twisting or bending long stock. 

I've almost stopped using it at all since building the (Naturally Aspirated Ribbon Burner) NARB forge.

Unless I haven't been clear, you want to build a REALLY TOO LARGE forge to be practical. You may use one that size once or twice before you take it apart and turn it into a couple practical size ones. Being a giant brick pile forge is the best idea you've had for this build. I'm not dissing you, most of us who've been at this a while have forges that are just to darned LARGE gathering dust somewhere in our shops. You will be able to take yours apart and make ones you can afford to use. This is a GOOD thing. 

Frosty The Lucky.