A chain stitch can be a sewing, knitting, crochet or embroidery stitch that is formed of interlocking loops forming a chain of stitches.

The chain stitch is an embroidery technique where many loops of thread are drawn together to make a chain-like pattern. You bring the needle out at the top of the stitch line, and then holding the thread below the stitch, insert the needle again INSIDE the previous stitch (at the top of the stitch) and bring it out again above the thread. This will form a chain.

For a broken chain, insert the needle again OUTSIDE the previous stitch.

The 3sc means "three single crochets." When the pattern says "in chain," it means you are to perform the stitch(is) in the actual chain stitch of the row below as opposed to in the space created by the chain stitch (which would be called a "chain space.").

Any stitch, as the chain stitch, that uses loops in the pattern or process of working

A picot stitch is made with chain stitches. For a small picot, chain 3, then put a slip stitch in the first chain stitch. For a larger picot, do 5 stitches the same way. If you chose to change the amount of stitches, odd numbers are best so that there is a middle stitch.