We can't initiate a flight if the destination isn't forecast to be legal. We can launch to a different destination and then change, but it's not at all a common thing.
That's under FAA regs?
JAR ops- You can dispatch with two alternates and fuel to the most distant. Amounts to the same thing, you've got fuel to go to two airports with suitable weather, and will land at the destination if it's available when you get there.
Australian regs- Don't get me started!!