Bridge plates are the total weight of the vehicle plus total load in American tons (yes even for British Vehicles) rounded up to the nearest ton. The 3-2 weight would be the weight in tons (3) of a Jeep plus trailer plus load and the (2) is the weight of the Jeep plus load. The 3 would be above the 2 and all vehicles that normally towed a load, such as Field Artillery Tractor, would be thus marked. There was an official booklet issued to give the correct weight, but in practice it seems to be whatever the Officer said it should be.
To give you some idea, a WOT2 is 4592 pounds plus 1680 pounds load, which is 6272 pounds total weight. In American tons this is 3 tons 272 pounds, which rounds up to 4 tons. Ford WOT2 trucks in WW2 had a Bridge plate of 3 or 5, according to original photos.
This information is for Wartime vehicles. I don't know if this was changed post-war. Can anyone else help here?