I know parents at all those schools, if you want to pm me.
Quickest way from Stockel to Montgomery is by metro line 1, around 11 minutes. 39 is slower but still not bad, takes around 16 minutes.
Acacia is around 5k per year, expect 25 in a class except in acceuil for children born in 2009, for 2 to 12 year olds. Montessori House Brussels is around 10k per year, smaller groups, only 2 classes for 2 to 6 year olds. Acacia is likely to give your child more French than Montessori House. Acacia has a more stable and Belgian intake and a reasonable amount of French speakers in the Fr/Eng classes as well as expat children, it is run a bit like a local Belgian school, perhaps a bit stricter. Montessori House has a high percentage of non Belgians and non French speaking expat children but has much smaller classes, quite different to a Belgian school. BJAB is very unlikely to teach your children much French, more like a British prep school. There is also BISB which is only 10 minutes from Montgomery too, by reputation a bit less pushy, but at 3 years old, I doubt that would make any difference, both BJAB and BISB are similar size, similar intake, similar fees, both English only with French second language lessons.
If you are here more than 2 years, it really can be a good choice to go for local rather than private, the more your children pick up French , the more integrated they can be and more activities they can happily take part in. You also get very cheap extra-curricular activities which would cost 2 or 3 times the price in the private sector. Plus out or hours child-care known as garderie is also going to be much much cheaper and more flexible in a local school - for example one private international school I know charges 6 euro per hour for garderie, our school charges a little less than 1 euro per hour. The local system is very very different though, lots of play to an older age to 6 years old, but at the same time lots of structure, children do as they are told and do things as a class, also children are expected to become autonomous at an earlier age, not so much molly coddling.
It depends on your budget what sort of gym you are after. Aspria has 3 clubs, Louise, Arts-Loi and la Rasante and I know la Rasante has a paying creche and minimum membership fees for one person of 1800 euro per year. There are much cheaper options, such as European Fitness next to Stockel metro, as little as 650 euro per year so almost a thrid of la Rasante price, finally there is Fitnastic which is very near Montgomery roundabout, no idea of cost but imagine European Fitness sort of price. Neither of latter 2 gyms have creches, instead you can simply use a halte-garderie then go off to a gym, the one near us charges 8 euro per morning session and has plenty of expat children in it, despite it being an ordinary halte-garderie (drop-in creche) subsidised by the commune.