Attaching weld nuts is a projection weld due to the design of the weld nut. An attempt to direct power from a single transformer to three weld nuts at once and maintain a balanced current to all three would be difficult.
If one wanted to use one transformer to make multiple welds a cascade arrangement would be a better set up. Here the transformer is attached to all three weld guns. The weld control is a cascade arrangement that allows only one gun at a time to close and have current flow.
FOUR CASCADE CONTROL
In the above control four cascade welds can be programed independently. Each weld would made independently in sequence. This reduces demand on the buss and the transformer since this is four individual welds not four at one time.
Reference: RWMA Resistance Welding Manual 4th Edition