Cheers! You are correct!
Give me the one that you truly love; - Story is King Graham had to give up Rosella to the dragon to save the Kingdom
And I will keep them away from you; high above. - The dragon lives up in the clouds
Until another, who is strong;
Decides to come along. - Gwydion/Alexander, coming along.
They who bring thunder and lightning; - The thing that beats the three headed dragon - the lightning spell.
The only thing, that I find frightening.
I always found it weird that it was lightning that defeated a three headed dragon that has it's home in the clouds.......