Reporting from Multiple Tables

I assume that you have three relationships from the parent table down to each of your 3 mile stone tables. I suggest that you make a summary field to summarize the minimum or maximum of the date of the mile stone and that will get that date up to the project record where you can show it on the report.  
NNow that assumes that you just want to float up a single date from the milestones table. If in fact you want to float up multiple dates from the mile stone tables then he will need to make a text field which is a formula like this

ToText([my date field]) 

.... Can use a combined text summary field to float up all of the milestones text dates up to the parent record.  You don't want them to appear in the combine text bubble format then you can make a new field like this  

ToText([My Combined text summary field]) or else like this to give a vertical list

SearchAndReplace(
ToText([My Combined text summary field]), " ; ", "\n")