How to find out a date based on specific value in different column?

Hi, here is the sample data set with available appointment slots of different doctors. The logic we need to apply is, starting from today we need to find the next third avialbility slot date irrespective of doctors. 

"Star date" column is the doctors appointment available date and "Begin time" column is the available appointment slot times.

For example, from the sample data set, the Red marked box would be our 3rd available slot and Blue marked box would be our 3rd available slot date(which is required).