Is it the RER from Paris Metro station that goes to CDG, ? I need to get to Terminal 3 for my departure .
Is it clear where to go when you arrive CDG ?
Yes and No.
Yes, the RER goes from central Paris to CDG from these stations: Gare du Nord, Châtelet les Halles, St Michel/Nôtre Dame, Luxembourg, Port Royal, Denfert-Rochereau, Cité Universitaire.
https://parisbytrain.com/charles-de-gaulle-airport-cdg-to-paris-by-train/
No, it's not clear ( to me anyway) where to go once you get there, but we figured it out, so I'm sure you will too.
You will have better luck getting this question answered if you post in the "France" forum.
Enjoy your trip.
https://easycdg.com/airport-guide/travel-between-terminals-move-around-paris-cdg-airport/
The RER terminal is downstairs Terminal 2 (it's a big building).