afdmello,
Select the two shapes you want to disappear. Activate the custom animation
pane, apply an exit animation using the add effect button. The first exit
animation should have an on click beginning the second should start with
previous (if you select both and apply the exit animation this should be the
case automatically). Now select the third shape, apply an entrance
animation, next to begin select with previous. When you click the two object
will disappear and the third appear.