If you want to use a specific single unit for your durations as is common in science (and partly in engineering) this isn’t a question at all. You have to enter numbers regarding the unit (which will then be implicit) and to calculate with them. To interpret the results for the real world you need to regard the unit again. This can either be the h or the min or the s or any obscure unit I never heard of. It’s the same as with any quantity and even with currency.
The specifics concerning time - and this for TOD as well as for durations - are due to the unfortunate habit of using mixed units, and specifically those conflicting with the otherwise used decimal numeric system. If you want to observe this habit you need to accept the one implicit unit of time for which it is supported: the day. For entering durations you will also need to pretend to not know the fundamental difference between TOD values and durations. This accepted you can enter a duration of 123 h 45 min 32.1 s
as 123:45:32.1
(in a decimal-point locale). Smart Calc will even automatically assign the ‘Numbers’ format code [HH]:MM:SS.00
to the cell - again mixing up TOD with duration. However you may change that to [HH]" h "MM" min "SS.00" s"
if you don’t want to surrender.
Since the actual content of such cells is always an ordinary number based on the unit d (day) the calculations are as with any numbers. Just calculate your =AVERAGE(A1:A12) and format the result in the appropriate way.
If you are a purist and insist on entering durations in a multi-unit-duration-format, 1h12min3.2s
e.g. it gets complicated. I cannot think of a way to achieve this without resortiung to user code.