Sum Rows from Different Tables, Deduct from Start Value

You can do this using the query editor and then a DAX column for the End Value:

1. In the query editor select Merge Queries as New
2. Use Table1 as your top table and Table2 as your bottom table and select the Project number on both tables. Select Full Outer as your join type.
   
3. In the new Merge1 table hit the double arrow button next to the Table2 comlumn heading and click OK on the window that pops up:
   
4. Now select the Merge Queries option (not Merge Queries as New) and join the new Merge1 table with Table3 the same way you did on the first merge:
   
5. Repeat step 3 on the Table3 column
6. Delete the 2 extra project number columns and rename the other 2 columns
   
7. After you close and apply, create a new column in the Merge1 table using this DAX:
   End Value = Merge1[Start Value] - Merge1[Value Day 1] - Merge1[Value Day 2]
8. Output: