unix sort for 2 fields numeric order
===

I need to sort some data with unix sort but I can't figure exactly the right syntax, the data looks like

    3.9.1 Step 10:
    3.9.1 Step 20:
    3.8.10 Step 20:
    3.10.2 Step 10:
    3.8.4 Step 90:
    3.8.4 Step 100:
    3.8.4 Step 10:

I want to sort it using first the major number, then the step number, e.g. the data sorted above would look like.

    3.8.4 Step 10:
    3.8.4 Step 90:
    3.8.4 Step 100:
    3.8.10 Step 20:
    3.9.1 Step 10:
    3.9.1 Step 20:
    3.10.2 Step 10:

I have found the way to sort by first number on this site:

    sort -t. -k 1,1n -k 2,2n -k 3,3n

but I am struggling to now sort by the 3rd column Step number without disturbing the first sort