I have to make it in three hours from landing to arriving at Gare de Lyon. I think taxi will be fastest, since I am not familiar with RER. It is on a Tuesday at 11:30AM that I arrive. Do you think three hours is sufficient? If I miss the TGV from Gare de Lyon, is availability for most trains available that day, say the next train? I don't have a flexible ticket. Thanks for any help.
If your plane arrives on time and you already have your train ticket in hand, then you should make it. You need to allow up to one hour to clear immigration/passport control and another hour to get to Gare de Lyon. However, there may be a line at the taxi ranks at CDG which will slow you down. For detailed information, including photos, on how to get to Paris on the RER, see the Paris by Train site. Take the RER B to Gare du Nord, transfer to the RER D, and take it to Gare de Lyon. The fare is €9.25. If you miss your train, seats most likely will be available on the next one. However, if you have a discount fare ticket and it is limited to a specific departure time, then it will be worthless and you'll have to buy a new one at full fare.
At that hour, if you arrive in time and get through immigration quickly, you can probably do as well as a cab with the Air France bus to Gare Lyon. You pay the driver in cash. THe bus goes directly to the train station. I once made an 8:30 a.m. train this way with a 7:00 a.m. landing, a checked bag, and a wait to buy a ticket at Gare Lyon. By the skin of my teeth--I do not recommend counting on that! But if all goes well, you can take the bus or the RER.
There are four Air France bus routes. The one you would take is #4. However, be aware that the bus runs every 30 minutes. If you barely miss one bus, you'll have a 30-minute wait for the next one.
I took the AF bus from CDG to Gare de Lyon and this is an easy bus ride which takes less than one hour and is much less expensive than a taxi. You only need to arrive at the train station 15-30 minutes before departure to read the board and know what platform you train leaves.