But they told our showbiz show Newsround Lite that they're now so used to performing that they'll be excited, not nervous.
"I like that bit of nervous excitement before I go on it gives me a bit of drive," explained Sinead.
And David agreed: "I'm just enjoying myself, now. The nerves are good nerves," said the singer-songwriter.
Meanwhile Lemar was totally relaxed about performing in the showdown.
"Going to be my usual laid back self, gonna chill, make sure I rehearse and learn all the songs," he said.
Leaving the Academy