How many cubic metres in a tri-axle dump truck?

As many as it holds. There isn't a set specification for this, and the bed/cargo compartment capacity will often vary in accordance with the commodity being hauled. E.g., a truck bought solely to carry mulch will carry more cubic yards/cubic meters than a truck bought to carry stone aggregates and/or asphalt, since the mulch is a much less dense material. It'll vary in accordance with that and also with the weight/length laws of the country you're in (which we have no way of telling).