Weaver cars are very easy to convert either way. In your case just remove the body mounted coupler and the 2 rail truck. Then get a hold of a set of 3 rail Weaver trucks and 3 rail Weaver couplers. They are for sale separately. The coupler just sits on top of the truck and once you screw the truck in it holds the whole assembly together. I did this on three different boxcars for a friend of mine who is a 3 railer.
Of course now that Weaver is going out of business getting these parts may be harder in the future.