Averaging times that are not in a range

I am trying to get an average response time for numbers that are not listed
in a range. They are formated as a custom format like this: h:mm:ss. I have
been able to get a range to work, but not durations that have other cells
between them. Below is a picture of my table. The three center times,
Dispatch Time to On Scene, are the times I would like to average. Can you
please help me with the correct calculation? Thanks!

Tk-1 - Sta 1 - 96 Acoma
Dispatch Time to Enroute 0:01:29
Dispatch Time to On Scene 0:06:00
Total Time-Dispatch to Available 0:29:15

E-2 - Sta 2 - 2065 Kiowa
Dispatch Time to Enroute 0:01:26
Dispatch Time to On Scene 0:06:14
Total Time-Dispatch to Available 0:33:30

E-3 - Sta 3 - 3620 Buena Vista
Dispatch Time to Enroute 0:01:17
Dispatch Time to On Scene 0:05:47
Total Time-Dispatch to Available 0:39:24