The Monstro-Coupler 3 fits 3" tube, outside diameter of 3".

The Monstro-Couplers are designed to fit pipes larger than the standard theatrical battens. CNC machined from solid billet aircraft-grade aluminum, they are perfect for attaching to odd size pipes and tubes. Monstro-Couplers have a narrow size range of application. Monstro-Couplers are designed with a plus or minus 0.0625" tolerance on their nominal size to ensure that they will fit with the Aluminum Association pipe and tubing manufacturing tolerances.