to me this looks like a typical stair step table design.
T2 (1) to T1 (many) joined by Emp ID
T1(1) to T3(many) joined by Task ID
An employee has many tasks, and each task has many sub tasks or steps. Quite common.
I believe if your starting point of reference is T2 then in your formula preceed the T3 Date field with the RELATED function and intellisense will allow you to find/select the T3 Date field.
One of the nice things about Power BI is you don't have to flatten tables - but if it makes things simpler you can merge the T1 Emp ID onto the T3 table.... and then just work with T2 and T3....but not sure this is important as it won't change your formulas.