To crochet, a treble crochet starts the new round by casting on 3 chain stitches. Depending on the pattern, these 3 chain stitches count as the first dc. If you work in a chain stitch, you will crochet your first double crochet in the fourth chain stitch from the hook.
Wrap the thread around the needle. Pass the needle through the stitch. Wrap the thread around the needle again. Then you only pull this thread through the inserted stitch.
You now have 3 loops on the hook. You wrap the yarn around the hook and pull the yarn through two loops.
You now have 2 loops on your hook. Wrap the yarn around your hook again and pull the yarn through two loops again.
Now you have one loop left on the hook, and your treble crochet is ready. You will now see 1 stitch and the first 3 chains you have cast on.
If you start a new line, you will get the correct height again by working 3 chains again and turning your work.