My engineering (???? lots of big questions marks because I am not an engineer) is done by seat-of-the-pants and gut feeling. And both of them are saying do not drive your five ton tractor, let alone your ten ton tractor over your three 10" I beam bridge at least until after you have written your will, assigned an executor and got all your affairs in order.
For bridges a good overload factor is maybe 5x so you should build something that can support a static load of 25 or 50 tons. That way when your 5 or 10 ton load bumps and bounces over it you do not overload it with the shock loads.
Find a reference website that will give you tables for the value of I for standard I beams and another site that gives you the formula for calculating the midpoint deflection of a simply supported beam and do the calculations. If your bridge is going to deflect more than about 1" with the 25 ton load I am not going to drive your tractor over it.
And if you don't know what I is or what a simply supported beam is don't build any bridges until you do.