First scan your image with a flatbed scanner or download it from web. The resolution of image should be 72 pixels per inch (PPI) or scan them to 300 PPI if you want to print the collage.
Now click file and new to create a file. Decide the size in pixels or inches, choose resolution in pixels depend on whether you want to print it. Choose background color white or transparent or color that appears in bottom of tool box.
Open three images by click file then open and select file from the folder. First select new blank collage and copy and paste first image in it. The image appears as new layer in Layer box.
Copy and paste the remaining two in new collage file. Now it appears in layer box. Drag layer up or down to adjust the three images which come on top, middle or bottom.
Change size of three images with transform tool. Select image in layer box and press Ctrl and A to make selection active. Then press Ctrl and T to enter transform mode. Press and hold down shift key to dragging out corner to increase or decrease the scale of image. Press enter when change made according to your choice.
Now save collage file as a Photoshop file or .psd if you wish to alter it later.