For international trains from / to Belgium you can use https://www.b-europe.com/EN (page for international travel of the Belgian train company).
It will show all options (different companies and high-speed / normal trains).

For Amsterdam to Brussel, make sure to look at the conditions of a ticket before buying the cheaper ones. A lot of them tie you to a specific train (time) these days.

For Brussel <> Luxembourg there is only one fixed price (and you are free to take any train on the chosen travel date). But IMHO Luxembourg only is not worth spending so many hours on a train.