Save the value just means put it in a variable. So put the number of the first button pressed into a variable called something like firstnum (so if it's button number 3 set firstnum=3). Do the same for second and third buttons into secondnum, thirdnum.
Then the real tracknumber = firstnum100 = secondnum10 + thirdnum.
It's a crude way of doing it but it should work. And when that's working and you understand it you can think about neater ways of doing it.
Steve