Should I multiply maximum tyre load by number of wheels to get maximum bike load?
It is unlikely that the “weight” listed is the capacity of the tire to support weight, but more likely the maximum internal air pressure in PSI (pounds per square inch). 

If it is weight capacity, then yes, each tire will support 75lbs, so a balanced load over 3 tires would support 225lbs. That assumes equal weight on each tire, though, which is difficult to achieve. 

Edit: 

Since the weight listed on the side of the tire is actually 175lbs, that is much more realistic as a max weight capacity per tire. That would make the total supportable weight for a balanced load 525 lbs.