If you look under your truck, provided you have the stock exhaust, the muffler has 2 pipes that enter it coming from the manifold and cats etc. Where they bolt up to the exhaust is a flange. I believe this is a 2 bolt flange, meaning there are 2 bolts that tie the two parts of the exhaust together. A 3 bolt flange uses 3 bolts.
In what I was saying with the cats that I have they already have 3 bolt flanges welded on.
Hopefully this will explain.
I will take a picture of the cats that I have and post up and you will be able to see them.
Okay, here they are
3" high-flow cats for sale - Tundra Solutions Photo Gallery
3" high-flow cats for sale - Tundra Solutions Photo Gallery
You can see there is an end welded on, this is called a 3-bolt flange. This flange is welded on for some installations and not for others.