@Mumtothreeandadog As CrochetBug says, there are 4 stitches down the left hand side. I've edited your picture to try and show this more clearly.
When you know what you are looking for, treble crochet stitches are easy to count - you are looking for the posts of the stitch, which I think of as legs, joining the ring of v-shapes at the top to the row underneath of the stitch.
Looking at it like that, I've numbered the posts for you (including your chain 3, which counts as a stitch), so hopefully you can see where you have one too many.