If you have 3 pictures and want them to be spaced evenly across the page you can do this easily. You will first need to select all the objects you want spaced.
Select the first object, and then use the shift key to select the other objects. Once you have all the objects you want spaced selected, click on the Align button and closer to the bottom is Distribute horizontally or vertically.
This does not also align them just distributes them with even spacing across the slide. If they are all different sizes this can look funny so the next PowerPoint post is on making all pictures the same size.