Textures can be animated though I think 3 frames will be too little to actually see anything(ie too fast). Do you want the textures to merge one into the other or just abrutly change like a slide show? Pure and simple way is go into the material editor for the object. At the bottom are animation controls, move to the frame you want and change the texture. How it behaves depends on what you want from my question above. Also before animating any textures save a clean file to put away. Once textures are animated they are very difficult to remove the animation....it's been a long time since I've done any of this stuff, think I have it right.