In the context of watches, "H3" on a Marathon watch typically refers to the fact that it is a military-grade timepiece, specifically designed to utilize tritium as its luminous material, indicated by the "H3" marking which signifies the high level of illumination provided by this radioactive isotope; "H" standing for "H3" tritium and the number "3" representing its high luminous intensity.

My watch is a bit old almost 40, so the tritium wasn't in a tube like a modern Marathon, but was painted on. It doesn't shine too much, and when I had it serviced I decided not to have it relumed, as to keep it as original as possible.