to the OP. It is very difficult to get data on the average amount of college debt because parent oftentimes will get money from other places (read home equity). The data you will find is generally what the student is on the hook for.
With multiple children you really need to make sure you are fair to each child. It is not appropriate to blow the load on the first one or even have the attitude that you will figure it out as you go. You need a plan. Can you afford the state flagship for all three? If not, then the bar has to be lowered. Maybe it is CC for all three. You just need to run the numbers.