3 Tons (tons/sq. inch) is a 1954 Rules of Proof term for a maximum service load of 8,938 psi and was standard for 2 1/2" shells
Great Britain is a member of
CIP - Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives and the H&H loads are "standard proof" meaning service pressure 740 BAR = 10,733 psi; maximum statistical individual pressure
850 BAR = 12,328 psi; proof 930 BAR = 13,489 psi
Without sending 5 shells to a testing lab, only H&H can tell you the pressure.
There are a few factory load pressures at the bottom here
https://docs.google.com/document/d/1...UOZEFU/preview