Hi JR8,
While there are a few
International Schools in Singapore that cater to only younger (e.g. 2-11 years) or older students (e.g. 12-18 years), the majority of schools offer a full range of primary and secondary education from age 3-4 to age 17. In that sense, most of Singapore's 29 (at last count)
International Schools that offer English schooling could be viable options for your three children to attend together.
Waitlists can be a major obstacle at some of the more popular schools, so it's definitely worth contacting schools asap to find out whether they have waitlists for each of the specific year groups that your children will be entering. If you are only willing to consider schools where all three of your children can definitely get a spot, then this process may help to significantly cut down your list of possible schools.
You may wish to note that some schools offer sibling or family discounts for families enrolling multiple children in the same school. These discounts vary widely, sometimes kicking in with the 2nd child, sometimes with the 3rd. Also, they may apply to different elements of the fee structure (e.g. application fees, tuition fees, annual development levies). Once you've established a shortlist of schools, it's worthwhile finding out from individual schools what their policies are in this regard.
Double bonus.