This treble filmstrip template is designed to fit our 95mm x 280mm acrylic rectangle. Simply download the template using any design software that can work with layers, make sure the template opens 96mm x 281mm so its slightly bigger than the acrylic, import your 3 photos and move them to the back layer, size them to fit within the template and print the design.
 
You can find our 95mm x 280mm rectangles here
or the freestanding mdf versions here