[...] the robot weight when it is loaded and unloaded(that can be done by adding/subtracting the weight of the load to the robot weight )
and also the WHEEL LOAD WHEN LOADED AND UNLOADED
Are the two things different [...]
Yes, the robot weight is the entire robot, while the wheel load depends on how many wheels and how the weight distributes over each.
Say you have a 4 wheeled robot with the load centrally placed - the wheel load will then simply be the total divided by 4.
If the load on your 3 wheels are equally distributed, you divide the total by 3 of course, but chances are, that the load on the rear wheels are different from the front wheel.
For selecting a wheel, only the wheel load is needed.