Recomer...This is a traditional way to secure drums after service work and, yes, during vehicle assembly, too. Some manufacturers find it expedient to use push-on spring washers that get installed once, become a chore to pry or break off, and prove impossible to find on a Sunday at any auto parts retailer! I buy these push-on brake drum spring washers in bulk through Dorman sources, sized for wheel bolt studs. Typically you get a box of 25 (minimum).
AMC apparently had an ulterior motive with these three screws. Note that the screws are not 120 degrees apart, so they index the drum in a specific position. If you install these three screws, the drum will always mount in the same position on the hub—and likely for good reason.
See the balancing weights attached to the brake drum? Did AMC balance these drums with the hubs attached to get a true hub/drum balance? (This could account for the extra stamina and centering effect of these three screws versus spring washers.) The screws assure installation of the drum in the "correct position". This would be useful when balancing the wheel/tire assemblies, either off the car or on the vehicle, using a floor balancer. During wheel/tire rotations or a wheel rim replacement, the brake drum and hub would always be in balance. The tire/wheel, if also balanced correctly on a tire/wheel balancer machine, could be installed in any position on the wheel studs with reasonable balance assured.
This is conjecture but sounds good, right? The obvious AMC aim was to get the brake drum on the hub in the same orientation every time.