Three weeks in Europe.

Maybe trains are better if you're staying within the country, but they're expensive as fuck and slower than planes. Even with added security and the extra hour to get to an airport via public transit, the 90 minute flight far outweighs the seven hour train.

You'd most likely fly out of Orly, not CDG, if you were flying from Paris to a regional destination. For anything less than 4 or 5 hours then a train is probably ok, but flying is the faster and cheaper alternative for most travel between countries.