Mike, the U36C you referenced is an MPC model that has an open frame AC motor versus the DC can motors. I've never had any pulling problems with my MPC era locomotives, though it goes without saying, you'll want to check the motor brushes and make sure you have fresh lubrication. That engine was manufactured in 1979, so it is now 40 years old. If it was well cared for and maintained, it'll serve you fine.
As far as your RS-3, most of the recent ones have come with a single motor and have a plastic frame versus a sheet metal one. So you must have had an RS-3 from a mid-1990's or earlier set. Again, I've never had any serious trouble with dual DC can motored engines. Did you by chance buy that used? Maybe it was missing the weight that is included inside the locomotive shell.
The Mabuchi can motors used in many of the traditional starter types of locomotives is hardly a precision motor the way a Pittman one is. They are though, reasonable for what you are paying. But once in a while you can get a dual motored engine where one of the motors is seriously dragging, or out of speed synch with the other motor truck. That happened to me once, and I ended up replacing the motor, and the engine ran much better.
And as with any locomotive, regardless of the cost, you want to make sure the gears are clean and you have a fresh lube on it and that the traction tires aren't worn and/or are slipping.
Adding some additional weight inside the shell of the engine on the frame (wherever there's room) can also improve your pulling power.
