Connect 3" BSP tapered (R3 / BSPT) to 3" NH fire hose thread (NST)

R3 and 3-6NH are both called the same nominal size but are different thread forms: R3 is tapered BSP (BSPT) at a 55 degree flank angle and 3-6NH is gasket-sealed NH hose at a 60 degree flank angle (87.884 mm versus 92.02 mm major diameter). They start a turn or two then jam without sealing, so a machined tapered BSP (BSPT)-to-gasket-sealed NH hose adapter is required, not tape.

The matching 3 inch label is the trap. The 55 degree versus 60 degree flank angle means the flanks never fully mate; add the correct adapter rather than forcing the joint.