With three children, many will give a discount (EtonHouse, AISS, ISS and I believe maybe UWC as well - though not sure), but this will only apply to your third child. In other words, you will still have to pay full price for your first two children. And the discount will not be much - perhaps 10-20% off.
The Canadian school will negotiate on fees, apparently. It is totally at their discretion and will be taken on a case-by-case basis (income details need to be provided).
Icelady: I think that costs are a very relevant point when we are talking about sums that are in excess of $45,000 a year - which is EASILY what the OP would be faced with for three children. More and more people are here without companies paying out for schooling, and schools know this.
To be honest, with your younger children, I would look for a local-based kindergarten. Kindy's go up to the year a child turns 6. There are lots here that are run by expats and are staffed by expats, so they are very multi-cultural and yet, not too much of a culture shock for your kids. They are often significantly cheaper than
International Schools too.