Paris Itinerary Help

I am debating on adding Amsterdam and Brussels to my trip (2 nights each) and the rest of the time in Paris. Does this seem like a good idea? or should I just focus on Paris?

No, the going to Amsterdam part does not seem like a good idea since you say you don't want to be on a train more 3 hours at a time. It will take that long to go there or to get back from there to Paris. Google Belgium tourism and see if there is any place there other than Brussels that interests you that is not too long a train ride away. For example
https://www.travellerspoint.com/guide/Belgium/

I've looked at getting to other places in France, but travel time seems long from place to place, I don't want to be on a train for more than 3 hours at a time. Anyone have any suggestions?

You should get a guide book to France to see if there are places close by that you are interested in seeing.
Or Google Daytrips from Paris and see where you would be interested in going for example
https://www.thecrazytourist.com/15-best-day-trips-paris/

Also, I stayed in Marais last time and was thinking about staying in Montmartre this go round. Good idea or too far from everything? I'm interested in cemeteries, churches and architecture.....oh, and good food

Look at the map and consider where you want to go in Paris and determine if Montmarte is too far from those things.
Montmartre has a cemetery, churches, and old buildings and restaurants that have good food.

What time of year?

I think it's a doable idea but I would select either Amsterdam or Brussels. I favor Amsterdam. From Gare du Nord, the train on Thalys is 3.5 hours which is just over your 3 hour limit. I will say that I enjoyed taking the Thalys two months ago between Paris and Dortmund, a longer ride. I typically don't like being on trains more than 3 hours either but Thalys was just fantastic and that was in the 2nd class carriage.

Amsterdam is a great place to visit for 4 nights and the one I would select as my base.

Trip will be in October. I should mention that my flight is not booked yet and that if I add Amsterdam and Brussels, I would fly in there and fly out of Paris.

October is a great time of year! If you add both, you will be spending your precious 10 days moving to 3 different hotels. That's up to you.

I would spend the entire time in Paris, but then we like to settle in somewhere and explore. Take a day trip or two; wander areas less frequented by tourists (St. Denis, Chateau di Vincennes); get an apartment and live like a (rich) Parisian. There are soooo many day trips from Paris!

If you feel you have to add on, add only one other destination. Three spreads you so thin.

I would fly into Paris and fly out of Amsterdam ( it’s a much easier nicer airport for departures ) - I would do Amsterdam only - Brussels didn’t overly impress me , but I know some people just like to add countries to their “ seen it list “ .

I would give Paris minimum of six nights , and Amsterdam a max of 3, it may be cool and rainy by late October in Amsterdam and therefore days wandering the streets don’t seem quite as attractive ,but you do have an extra night so obviously it’s not too hard to tack it onto either place

My family and I did Paris and Amsterdam as a vacation and it was wonderful. We enjoyed the high speed train and I agree flying out of Schipol is a breeze compared to some other airports. We actually stayed in Haarlem but loved our experiences in the sites, food and people of the Netherlands.

You may have already visited some, but if you are interested cemeteries, I highly recommend Père Lachaise cemetery in Paris, and of course the American Normandy Cemetery in Colleville-sur-Mer. The latter will take more than a day to visit IMHO. Also, the Montparnasse cemetery in Paris is nice to visit (I visited Serge Gainsbourg's grave there).

Thanks PharmerPhil! I missed all the cemeteries my last go round in Paris, it was my first time there and I was trying to see "the highlights of Paris". I'm excited to make this trip revolved around them. Thank you for the suggestions.

I love Amsterdam and would second Pat's suggestion of flying into Paris and flying out of Amsterdam. If you're interested in seeing Belgium, I highly recommend Bruges. I've been there twice and I love it! In my opinion, Brussels is good for a day trip but I wouldn't necessarily stay overnight there.