Calculate values based on difference between 2 Dates

Tarun-Bhatia , One way is populate date from one table 2 another , asuume order no or some key is there

new columns 

Table 1 Actual Delivery Date = maxx(filter(Table2, Table2[order no] = table1[order no]),Table2[Actual Delivery Date])

Table 3 Actual Delivery Date = maxx(filter(Table2, Table2[order no] = table3[order no]),Table2[Actual Delivery Date])

Table 3 Planned Delivery Date = maxx(filter(Table1, Table1[order no] = table3[order no]),Table2[Planned Delivery Date])

You can take date diff as columns now .

Or refer how to take across table date diff

https://community.powerbi.com/t5/Community-Blog/Decoding-Direct-Query-in-Power-BI-Part-2-Date-Difference-Across/ba-p/934397#M451

refer if need

refer 4 ways to copy data from one table to another
https://www.youtube.com/watch?v=Wu1mWxR23jU
https://www.youtube.com/watch?v=czNHt7UXIe8