How to add multiple journey to same day while purchasing the pass in the planner

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

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.

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.

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.