How many tons can a tri-axle dump truck hold?

It varies by state, and other factors. In North Carolina, 17 or 18 tons is typical, dependent on the vehicle's tare weight and wheelbase, whereas dump truck drivers in Pennsylvania and Delaware have told me they were allowed to gross in the vicinity of 72,000 - 73,000 and could haul 23 or 24 tons with that. In Colorado, you don't see tri-axles very often, as the laws here only allow additional weight if it's a tag axle which extends the vehicle's wheelbase.