In your example you could set milestones at Start + 100 days, Start + 200 days and Start + 300 days. This could be accomplished with a predecessor on each milestone. For example, let's say your project start is a milestone at task ID 1. Then the first "progress milestone" would have a predecessor of 1FS+100d. That would set the first milestone 100 working days after project start. If you want your milestones in calendar days, the predecessor of the first progress milestone would be 1FS+100ed (elapsed days).
Bear in mind that milestone are NOT tasks, they a simply a point in time with zero duration.