You only need to scan in to one fastpass in order to schedule more at a different park. So, once you use the first one at AK, you can then go on then and make ones for the other park. You can even make a second Tier 1 at the different park before using the first 3 at the initial park (assuming one of the ones you use in AK is a Tier 1). You just can't make a second Tier 1 at AK until you use all 3 of the initial fastpasses.
It's explained pretty well in the FP addendum sticky, a couple posts down on the first page.