I don’t know what border extensions are but to join 2 pieces using a 3 needle bind off here is what you do. Put the live stitches of each of the sides to be joined on separate needles (they can be dpns, or the two ends of a circular as well). With the left hand hold the two needles up next to each other with the business end of each at the same end (like you would any single needle you were going to knit from) and the knitted pieces must have the right sides together.
Insert the right hand needle as if to knit (because that is what you are going to do) into the first stitch on the needle nearest you and on though the first stitch on the back needle as well. Yarn around the way you normally would and pull up one new loop onto the right hand needle. Slide the two stitches off their repective needles. This has knit the stitches from each piece together into one new stitch. Now knit the next stitch on each needle the same way. Now you have two stitches on the right hand needle. Lift the first stitch over the second one like you would for any normal bind off. You have bound off one stitch and begun to join the two pieces.
Now knit the next two stitches together and bind off. Keep doing that across. End like you would for any bind off.
There is a video about how to do it on this sitehere. It is the last thing on the page.