I searched and know that here are 3 different mounts which are:

CA MP5/G3 low profile mount

G&G HK long low profile mount

G&G HK short low profile mount

So what one should I use for a Burris FastFire Red-Dot Reflex Sight?  

The MP5/G3 is out of stock and I dont think I need the long mount so will the short work?

Just get a $10 mount off ebay. You cant screw up a mount. There are abotu 12 threads on here with different mounts people are currently using.

Complete and utter B.S.
I just bought a $15 mount from M1Surplus off of gunbreaker.com, and had nothing but trouble with it.

First, it didn't fit right, and just barely held the tab lips in place.
Second, the rail does not extend the full length, and I could not get the Aimshot HG-M1 sight to fit it, so I tried the other cheapy I bought, a Bushnell Trophy MP, and it wouldn't fit. In desperation, I quit with the el-cheap-o's and put my A.R.M.S quick detach mounted Aimpoint M2 on it...AND IT WOULDN'T FIT.
"No matter", thought I, ever the enterprising individual, "I have a seperate rail lying around...how 'bout I drill this thing and screw that on top...should solve everything!"
I figured it was disposable at $14.95, and this could save the day. Wrong.
LITERALLY as soon as I touched the dremel to the underside of the base, one of the mounting ears cracked off. The casting was not even paper thin...it was even thinner, and I shudder to think what might have happened if I had taken this thing into the caves of Tora Bora!
It went into the dumpster after that, and I had a few back and forths with M1Surplus, and was surprised how enthusiastic they were in trying to help me and make it right. They were even willing to take it back for a full refund even after I told them I had been drilling it to make it work. I declined, largely as I knew it wasn't their fault, and I didn't want to go dumpster diving at 11pm in a rainy PacNW cold night. But their service was the only good thing to come out of the situation. I will buy from them again...Just not an NcStar low profile mount.
There are good, cheap ones out there, but this one is not one of them, and goes to disprove your previous statement as quoted above. I still haven't found it yet, which is why I'm here.

It's ebay, and therefore, a crap shoot.

I bought one of these.

http://members.cox.net/mailbox6/rail.jpg

It's solid as a rock. It clips right on, tightens down and fits well with no modifications at all.

I paid $8.00 for it, plus about $8.00 shipping from Hong Kong.

YMMV, but I'd do it all over again.