Would It Be Possible To Cut A 3" Hole Into The Side Of A 6" Main Stack C.I.
To Ventilate A Bathroom Fan?
Building Is Brick Block / Would Rather Not Have To Go Through Wall
Just To Vent A Bathroom Area.
I Could Run The Tee In About 2 Feet Before It Goes Through The Roof.

Thanks Any Ideas Greatly Appreciated

Can you do it, yes. Should you do it,no, it is illegal and could cause fumes to back up through the fan when it is not running.