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there may be a way to do it with powerPCB but the stanrdard way is to make gerbers and then merge them together. I use gerbmerge, it's free. It works quite well though the set up is a bit complex. It requires python to be installed and you run it in dosbox. http://claymore.engineer.gvsu.edu/~steriana/Python/gerbmerge/
use gerb tool. take the time to learn it. The amount of time you spend fussing with merging 3 boards in powerpcb will be more than the time you take to learngerb tool. Also, if you make changes to one board, re-merging the geerbers will be a lot easier than remerging the board designs.
All you need to do is make sure all the netnames and ref des. are different.
Export 2 of the boards to ascii making sure the origins are in the location you want the board to come in.
then import each into the main file
i did before a copy and paste maethod. Provided ur confident enuff & none of ur PCB need anymore checking. but just a reminder the file size is large.
As most recommended do use a gerber tool software e.g CAM350 and merge the gerber tgr.