Gare St. Lazare train to Caen

Take a look at https://www.sncf-voyageurs.com/en/travel-with-us/prepare-your-trip/accessibility/assistance-services-and-assistengare/ to see if they can help you. Is there anyway to get the baggage down to a one piece of luggage per person? That would help with boarding the train. The platforms are usually posted about 20 minutes before departure time. I have seen train platforms with a diagram posted showing where to stand for boarding but I am not sure if that is always the case. I don't think that Gare St. Lazare is really a pass through station meaning it is the end/beginning of the line for trains to Normandie so you are not likely to be dealing with rushing to a train that will be at the platform for only five minutes.

They normally post platforms about 20 minutes before departure. That is enough time to go there and board even for the slowest of persons

So do not overthink this, and do not worry.

My husband and I are in our 80s and have switched to medium size backpacks as our airline carry on luggage. For long trips we check a larger suitcase as well (carry on size or one larger 24 inch bag - so two not very big bags, one for each of us). With these we can easily get aboard a train with our luggage -- although for side trips from Paris we just travel with the backpacks which is great for train travel. 3 bags is hard if you are old and mobility impaired. Consider small day packs and then two not very big bags so each person only has to haul one aboard the train and each one can handle the size bag they are hauling. A carry on spinner perhaps each..

St. Lazare is the terminus/ origin of the train and so it sits there for 20 minutes or so -- boarding a train along the way can be a lot quicker, Be sure when you get to Caen you are standing by the exit door with your luggage when it pulls into the station; the stop will be adequate, but brief.

I would be more concerned about getting you and your luggage off the train in Caen. The train starts at Gare Saint-Lazare, so it will be parked for many minutes before departing. But it is apt to only stop in Caen a minute or two before moving on. They won't wait for you, and if you and two of your pieces of luggage get off, you might watch the doors close on your other piece of luggage (or your significant other) before it heads to points further west. I would concentrate on packing such that everything can be moved by the two of you on your backs or in your hands in one motion. Do research how to find your first-class car on the platform at Saint-Lazare though. If you get on the wrong car with three pieces of luggage, it will be very painful to try to move your ensemble through the length of the train to find your car and seats. Bon voyage!

Please take a look at the assistance proposed in the link provided kindly by JHK (in English).

My guess is that you would qualify for the service as what France calls "PRM" (persons with reduced mobility). You will note that to use the service, you can each have a suitcase of a certain size and weight.

The service is for assistance boarding and disembarking.

I strongly encourage you to downsize. We traveled from Edinburgh to England to France for over three weeks by train using two suitcases and my large shoulder bag. It can be done. I seem to remember it was a long trek at one of the Paris stations to our car. I also remember one of the trains had a platform when you got on the train and then you had to go up maybe four or five stairs to the seating area. Can't imagine doing that with three suitcases off and on especially if the train is crowded. We are in our late 70's and managed very well. You can do it with two suitcases. Good luck and enjoy.