To find flights into and out of different cities, don't price two one-way tickets. Instead, use the "multi city" option.
The answer about price is, it varies. My sister and brother recently went to Amsterdam and Brussels. She was able to get an open jaw (multi city) fare from Dulles into AMS and out of BRU that was not more expensive than a round trip. However, he was coming from Philadelphia, and for some reason the open jaw flights were about $150 more than round trip to Amsterdam. He and his wife ended up taking a train on their last day from Brussels to AMS to catch a flight back home.
And, while I'm generally not a fan of Brussels, the reason they went there was to see some of the artworks in a specific museum; they were very glad they did it. So, if you have a specific reason to go to Brussels, you certainly can. However, if you're just going because it's on the way, don't bother - the other two have much more to offer most visitors. The train from Paris to Brussels is about 3 hours, so there's no need to break up the trip for length.
Thalys runs these trains, and the sooner you buy, the cheaper the tickets (I forget if they go on sale 90 or 120 days before travel): https://www.thalys.com/