The British Ton = 2240 lbs, so 3 Tons = 6,720 lbs. As Larry said though the gun was proofed to a higher pressure & certified as proofed for 3 Ton pressure loads. The 3 ton reference was also a crusher pressure measurement & not the later PSI as obtained by use of Pieso-Electic crystals which will show a higher pressure than the crusher readings.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra