Hello
A party of 3 seniors will be arriving at Paris Charles de Gaulle on September 3rd at 0830 hrs. We need to know how to get to Paris st Lazare and if it is feasible to take a 10:20 train to Dieppe.

No. You'll never make it. I strongly suggest you wait until you reach Gare Saint-Lazare and then purchase tickets for the next train to Dieppe.

Agree with Bardo no way you are going to make it . You might still be at the airport when that train leaves!Plane might be late, you might not get off at a gate and have to take the bus from the plane to the e=terminal, then immigration might have long lines , they always have on our many trips, then baggage reclaim and getting to your next transport option. A cab or shuttle might run into heavy traffic or an accident

Uh uh, not going to work. Just get to St-Lazare by whatever means you think is best for you and buy tickets for the next train.

I'd like to add that I arrived at CDG last month at 7:45 am. I was sitting in a cafe in Montremartre by 9 am. I had carry on luggage and there was no line for passport control. I jumped in a taxi and it was very quick. Things that will affect your result: do you have checked bags? are there other international flights landing at the same time? will you be able to move quickly to the taxi stand (with three, just take a taxi). The safest thing is to buy your tickets at St-Lazare when you arrive. You may make the 10:40 train, you may sit at the station and enjoy a nice meal while you wait. Good luck!

true, it could happen, but the problem is that there is no way you can predict those factors (if other flights are landing at the same time so the passport line is very long). And obviously you cannot predict that your plane will actually land exactly on time.

To have a chance of making that train everything would hae to go perfectly:

Plane lands on time
Gate is ready, pull in quickly
Get off plane first -not wait for 250 others to exit
Walk briskly and get to immigration before others on you flight (who may be jogging)
No other planes land at the same time
No line at immigration
Your luggage comes out quickly
There is no line for a cab
You don;t encounter traffic or accident on the way into Paris

Are you willing to risk that?

I have had it take more than 2 hours to get through the airport, get luggage and get outside on taxi line (mostly due to mobs at Immigration). Not always, but it's happened more than once.