Oh okay so you need the sum total of those numbers to be formatted so that the separator is colon :
This is how I got it to work:

Since power fx is in preview there are some bugs. It for what ever reason is not able to convert it straight up like this: =Text(Sum(sum1, sum2, sum3), "00:00")

It can format with the text function like so: =Text(Sum(sum1, sum2, sum3), "00:")
But not if the formatting has colon and numbers after that like ":00"
So thats why you have to split the numbers and add them together again with colon as separator.
Split the sum of the numbers with . as separator. and take the Last item of it.
=Last(Split(Text(SumNumbers), "."))
returns:

Then format the sum with formatting "00:" and add the SumLast.Value
=Text(Sum(sum1, sum2, sum3), "00:" & SumLast.Value)
returns:
