Kate Middleton

Happy anniversary to the Duke and Duchess of Cambridge! Kate Middleton and Prince William are celebrating their third wedding anniversary this week, marking the years since they tied the knot on April 29, 2011.

The royal couple just returned from their three-week tour of New Zealand and Australia last weekend, and plan to keep things "low-key" for their anniversary celebrations, a source tells .

"They chose to make low-key plans but it will still be a special day for them," the insider told Us. "They decided to do something low-key and in private before the tour and have been looking forward to celebrating."

The parents to young Prince George, 9 months, are enjoying the rare downtime in London. A palace source tells Us, "As always, summer is a busy time for the couple and this year will be unlike any other. Their schedule will be very busy in June, July and August."

William, 31, married his bride, 32, at Westminster Abbey in a grand ceremony. While there is no word on the exact anniversary presents the couple will exchange, the traditional British marker of a third anniversary is a gift made from leather.

The couple's celebrations will be quick, as the Duke is scheduled to leave London with his brother Harry -- and is U.S.-bound! William is set to attend the weekend wedding of his close friend, Guy Pelly, in Memphis, Tenn.