That makes absolutely no sense. Both 1/2s of the winding are essentially in series with each other from start to finish. Current is not summed in series circuits (in regards to your statement about having 2 x 1.5A outputs).
Furthermore, the 3A rating is CONTINUOUS current. In a full wave grounded center tap config current flow is NOT continuous through each 1/2 of the winding. Each 1/2 of the winding has a 50% duty cycle in this arrangment, which allows both halves of the winding to source double the current, but at 1/2 the full winding voltage.