I can create a rounded rectangle and use the fx (fill/stroke) but I only want a shape with three sides. Because it's not enclosed the fill/stroke etc won't work. I can create all four sides and add another layer to mask the side I don't want but wonder if there's a better way to do this in Photoshop
Add a mask to the layer. Then right-click (Windows) or Control-Click (Mac) and choose Blending Options.
Then tick the Box for Layer Mask Hides Effects.