tm3/h?
I think all will agree that m3/h is cubic meter per hour. The t is the question mark. The t is often internationally used as abbreviation for ton, and accepted as symbol for this (1t=1000kg) together with standard SI-units. (Same for hour h=3600 SI seconds).
This is probably the nearest you can come to official standards. What tm3/h means in your context has to be explained by the person using the unit if it is not self-explaining.