Your sock looks great so far!
To put the stitches on three dpns, find the center of the heel and then put 1/4 of the total stitches on the left side; that's the 1st needle. Then put 1/2 of the total stitches on the next needle; that's the 2nd needle. Then put the remaining stitches (1/4 of the total) on the third needle.
For example, if your sock has 48 stitches:
Needle 1: 12 stitches
Needle 2: 24 stitches
Needle 3: 12 stitches
Now I've never knit with 4, only 3, but I like using 3. Not so many pointy things on which to impale myself!