Hi Bob,
Relatively easy. Easiest route would be the
Orlyval train between Orly and Antony station on RER B train line, then RER B up through Paris and on to Aeroport Charles de Gaulle 1 station (the first station), which is the closer of the two stations to Terminal 3 at CDG (about a 5 minute walk).
Best price for an Orly to CDG trip by train is 18€ through a Paris Visite carte, 6 zone, 1 day. If you bought single tickets, it would be 9,60€ into Paris and 8,90€ from Paris to CDG, a total of 18,50€.
Hope this helps,
Ben