Increasing a block at the end of a filet crochet row is tricky but still relatively simple. When you increase blocks at the end of a row, make sure that you don’t tighten up too much on your stitches.
Increase one block at the end of a row:
Yarn over (yo) twice.
Insert your hook in the top of the last stitch of the previous row, which is where you just worked the last double crochet (dc) of the current row.
Yarn over and draw the yarn through the stitch.
Yarn over and draw the yarn through 2 loops on the hook.
Repeat Step 4 twice.
You have 1 loop remaining on the hook and have completed 1 triple crochet (tr) and the second stitch of the block.
Yarn over twice.
Insert your hook near the bottom of the post of the last triple crochet (tr) that you made.
Yarn over and draw the yarn through 2 loops on the hook.
Repeat Step 8 twice until you have 1 loop on the hook (completing one triple crochet).
Repeat Steps 7 through 9 (completing the third stitch of the block).
After you master increasing one block, increasing more than one is easy. You just repeat Steps 7 through 9 three times for each additional block that you want.