I assume you mean that you will have 3 glass sides. In this case you need to get one door and two side panels (these can be fitted to either side since they just slot on and screw into each other). You can buy these separately at most places - b&q is a starting point, and can be got in a variety of sizes to suit whatever tray you're getting.
After that it's a matter of cost - cheapest i've used lately is from plumbworld.co.uk but you'll have to contact them direct to get them to order an extra side panel from the manufacturers.
If you type the key words of your question into our search box to the left of the site you may find the answer is already posted or is in the DIY projects section of the website. Every post goes through a monitoring process and using the search box may speed up your answer.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!