Trigger Pipeline Based on Formula Checkbox Changing

I think this is pretty easy. Just have a pipeline. Do a search every single day looking for records where the checkbox is checked, ie today is that 3 month date.  If you want to safety needed a bit, you can look for records that are three months or older which have not yet been processed with the follow on pipeline.  So you would have to add a step to the follow on pipeline to flag the record as being processed.