AFIK the 3 day pass is only valid for travel in France OR travel in Benelux, but not for travel across the borders. That means you would need the 4 day Global Pass, which allows journeys across all 33 countries for up to 4 days in a month, but only a maximum of 2 in your country of residence.

OOI to buy tickets for your journey plan would cost approx 200 - 300 euros using Thalys if purchased today (depending on your choice of train). There are even first class seats available at not much more on some trains.

With a pass Thalys trains carry a reservation fee of about 80 euros for your plans.

Just to add to what the other folks have already suggested, if you're only planning on doing those 3 train trips then buying point-to-point tickets might well be cheaper and less hassle than a Pass.

So that would mean buying a single ticket Amsterdam to Paris, for example using https://www.nsinternational.com/en. And a day return ticket for Paris to Bruxelles, for example using https://www.b-europe.com/EN.

A Pass sometimes offers more flexibility and better value when taking several trains in different countries for a less pre-planned trip. But since you want to take popular trains at a busy time and will likely encounter a need to reserve seats anyway, that aspect of the Pass becomes less relevant.

As @rvdborgt said, that tool is not much use.

It is telling you 3 day pass for 2 travel days because 3 days is the shortest pass sold.

That route; Amsterdam and Brussels to Paris is one of the worst to be trying to use a pass on as there are large reservation fees and small quotas for passholders.

I never bother with that tool and count the days myself...