Wow, what fun! I've never knit a mermaid's tail.
You've done the right thing in dividing the sts. A round counts as knitting 30sts or all the sts on all 3 needles. The halfway point is between sts 15 and 16 so if stitch one is on needle 1 and stitch 30 is on needle 3, sts 15 and 16 are on needle 2.
When you have the markers in place and you've knit one round, you're back to the marker before stitch one. Move the marker from the left to the right needle. Knit sts one and two together (k2tog). Knit to two sts before the next marker (sts 14 and 15) k2tog, move the marker to the right needle, k2tog, and knit to two sts before the end of the round and k2tog.
When you knit with double pointed needles and are using 3 needles to hold the sts, you'll use the fourth needle as the working needle. Take it in your right hand and knit the sts from needle one onto it.Once all the sts have been knit off needle one, it's free and it becomes the working needle.
There's a video for k2tog on the Glossary page if you need it. Here's a video for dpns that may make it clearer too.