Adding time

Hello, im trying to add a bunch of different times.

For example, i want to add the time in Textbox1 with the time in Textbox2
The time is in there like this 3:48, as in 3 hours 48 minutes.
I want to add those 2 up, so i get 7 hours and 36 minutes.
I want that to get posted in Textbox3 as 7:36
The problem is that i have to add like 20 different times, and will go above 100 hours in total.