If you did short rows for the Back shoulders...you must have put them (the remaining "live" stitches) onto either waste yarn or holders.
To perform the 3-needle bind off...you will need to have an equal amount for each Front shoulder as well. For example: all 4 shoulders have 16 live stitches on hold.
When the time comes, you will put the 16 for the Right Front shoulder onto a needle, and you will put the 16 for the Right Back shoulder onto a needle...and THEN you take a 3rd needle for the actual binding off process.
You prolly already know that, but just in case....
And, Knitting Help has a video
(it's the last one on the page) on how to work the 3-needle bindoff, but you prolly know that, too, but just in case....
And don't forget YouTube. Just enter 3-needle bind off knitting in the search bar
once you get there. Here is just one: