How much weight can a tri-axle carry?

Varies according to weight rating of the axle, overall wheelbase length, what type of road it's on, and state law. In Colorado, having an additional pusher axle does nothing for you, due to state law. In North Carolina, a tri-axle dump could typically carry 16 - 17 tons on the Interstate, and 10% more on a secondary road, depending on the vehicle's tare weight and wheelbase. Some states, I've heard of tri-axles grossing 70k on secondary roads. There really isn't one set answer to this.