Programming :: Transform Mm:ss:ff To Mm:ss.ss?

Bash 3.1.7

I have var1 in a script.

Code:
#!/usr/bin/bash
..........
echo var1
with ouput
08:28:10

The format of var1 is mm:ss:ff where ff are units of 1/75 seconds. In order to make of var1 an argument to the flac command (flac --skip=mm:ss.ss where ss.ss are seconds and decimal fraction) I must trasform mm:ss:ff to mm:ss.ss).

First I must isolate ff and place it in say, var2. And then var3=var2/75. How can I do the part (mm:ss:ff --> ff)? Perhaps some way to specify bash to accept ':' as separator?