Michael Phelps and his wife Nicole Johnson announce they're expecting a third ...

He's possibly the greatest swimmer to have ever lived.

Now Michael Phelps is about to expand his family with a third child.

The 33-year-old swimmer posted an Instagram photo on Sunday revealing his wife Nicole Johnson, 33, is expecting their third child. 

Baby on board: Michael Phelps, 33, posted an Instagram photo on Sunday revealing his wife Nicole Johnson, 33, is expecting their third child. The photo featured their two young sons, Boomer, two, and Beckett, one

Baby on board: Michael Phelps, 33, posted an Instagram photo on Sunday revealing his wife Nicole Johnson, 33, is expecting their third child. The photo featured their two young sons, Boomer, two, and Beckett, one

In the cute snap, Michael and Nicole's two sons, two-year-old Boomer and one-year-old Beckett, are seen driving a toy truck while the family dog tries to hop in the driver seat.

The black vehicle has balloons spelling out 'BABY' tied to the front grill.

'#3 on the way!' wrote Michael. 'Can't wait to see the journey that this takes us on!!!' He added the hashtags #luckyman, #boyorgirl? and #P3.

Third time's the charm: '#3 on the way!' wrote Michael. 'Can't wait to see the journey that this takes us on!!!' He added the hashtags #luckyman, #boyorgirl? and #P3; pictured together in 2016

Third time's the charm: '#3 on the way!' wrote Michael. 'Can't wait to see the journey that this takes us on!!!' He added the hashtags #luckyman, #boyorgirl? and #P3; pictured together in 2016

Sticking together: The star Olympian first met his future wife at the 2007 Espy Awards. They began their relationship in 2009, though it had rocky patches along the way; shown in 2016

Sticking together: The star Olympian first met his future wife at the 2007 Espy Awards. They began their relationship in 2009, though it had rocky patches along the way; shown in 2016

The star Olympian first met his future wife at the 2007 Espy Awards.

They began their relationship in

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