Yes you can apply. They require TRVs although some have now been required to get study permits. You will need to show that you have the extra funds to show that you can support 3 children. Not sure how old your children are but things like daycare or before and/or after school care can be very expensive. Working part-time won’t make sense because childcare would likely consume anything you earn. The main purpose of you being in Canada is for you to be a successful student so you’ll have to show how that will be possible as the single mom with 3 children.