In my opinion it's best to plan on the fireworks and Fantasmic on different nights. It just makes things easier. Plus, if there is wind and rain there is more of a chance of the fireworks getting cancelled than Fantasmic being cancelled.
But your options would be:
Watch the first Fantasmic then stay seated to watch the fireworks. You will hear the soundtrack and see the rocket blasts. But you will miss all the effects on the castle and the Matterhorn.
Watch the fireworks from in front of the castle, then make your way over to Fantasmic for the 2nd show. They won't let you in that area until after the fireworks anyway, so you will still be able to get a good view.
Watch the fireworks on your first night, then Fantasmic on the 2nd or 3rd. The later showing will always be less crowded and therefore a bigger chance at getting a good view. This would be my choice. And if for some reason the fireworks get cancelled, you still have 2 other nights to try to see them. If you wait until the last night to see the fireworks I'd hate to see it get cancelled on you.