I use 3 needle BO’s a lot, it is my favorite way to seam shoulders, it gives a neat, small and strong seam.
On top of how great it looks it also saves time, instead of binding off two sets of stitches per shoulder and them seaming I only need to do one 3 needle BO!
Step 1: Turn your garment inside out so the right sides face each other.
Step 2: Put both sets of shoulder stitches (or what ever stitches you are binding) onto the needles. and hold them one in front of the other.
If there is no working yarn attached, attach some to the right hand side, it doesn’t really matter which set of shoulder stitches you attach it to
Step 3: Get a third needle one size larger than the other 2 needles.
Step 4: Insert the larger needle into the first stitch of the front needle and the first stitch of the back needle as if to knit.
Step 5: knit the two stitches together.
Step 6: Repeat steps 4 & 5, so you have two stitches on the right hand needle.
Step 7: Pass the first stitch on the right hand needle over the second stitch so one stitch remains on the right hand needle.