Connect 3" BSP parallel (G3 / BSPP) to 3" NH fire hose thread (NST)

G3 and 3-6NH are both called the same nominal size but are different thread forms: G3 is parallel BSP (BSPP) 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 parallel BSP (BSPP)-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.