Enlarging water heater flue size

[Steve Wilde]

I have an old water heater that is slowly dying, and in need of replacement. The old water heater has a 3" vent, and the new will have a 4" vent. The vent goes into a common flue that the water heater and two furnaces all vent through, and goes all the way to the roof, two stories up. How would you go about enlarging the hole where the water heater vent attaches? Replacing the water heater is easy, but the flue has me a bit baffled. One idea is to get an oops arbor for my hole saw and use a 3" saw on the inner arbor, and a 4" on the outer arbor and drill it out, but I'm not sure the flue is rigid enough to do that. Any Ideas?
i added the photo, not sure why it's sideways, but the upper one is the water heater...

[Bruce Wrenn]

Why not replace the 6 X 3 tee with a 4 X 6 tee? As wall is covered with sheetrock, you can enlarge space to work with very little effort. Plus that's B-wall pipe, which means it's two layers thick.l

[Steve Wilde]

Dang, I really am an idiot! That hadn't even occurred to me... It even has a sticker on the side of the pipe so I know what it is, it's metal fab pipe. Thanks, that's why I asked!

[Brad Adams]

It's simple. You probably can't do it. If you already have two furnaces and a water heater on it, the flue isn't big enough at 6". Depending on the size of the two furnaces, with the increase in btus of the water heater it may be too small.