Nicole Richie & Joel Madden getting married

They've been together for more than three years and have two children. Now, Nicole Richie and Joel Madden are planning to make it official.

"Yep. i'm engaged. Very happy. Yeah we've been engaged for a while so your all kind of late on that. But Thanks for the hooplah all the same," Madden Tweeted Monday night, after news of their plans to wed went wide. He also expressed thanks to relatives who did not let word of the engagement slip.

He Tweeted: "P.s. thats why i love my family and friends. None of them are sources. All good tight lipped people who let us enjoy it! Thanks everybody"

Richie said Monday night during a taping of the "Late Show with David Letterman" that the couple are engaged to be married, a source told PEOPLE.

After the show, Richie headed to the New York restaurant Abe & Arthurs -- without her fiance -- for dinner. "She was glowing," says a source. "She was telling her table how happy she was."

Richie, 28, and Madden, 30, began dating in December 2006 and have daughter Harlow, 2, and son Sparrow, 5 months.