Simpson and Ross already have 5-year-old daughter Jagger together, and the Boyfriend hitmaker also has 12-year-old son Bronx with her ex-husband Pete Wentz.
The couple announced they were expecting a baby boy in May, when they posted a cute video on Instagram of them slicing into a cake filled with blue sprinkles with the help of Bronx and Jagger.
In the clip, Simpson told her kids "Here we go, you guys all wanna help me?"
They then all placed their hands on top of Simpson's as she used a knife to cut open the cake.
And upon seeing the blue decorations pour out of the bake, Simpson excitedly screamed with joy "It's a boy!"
The month before their gender reveal, the couple had confirmed their pregnancy in another social media post.
Simpson also revealed she and her spouse are supporting charitable organisation March of Dimes and their Mom and Baby Covid-19 Intervention and Support Fund to help provide relief for pregnant women during the ongoing coronavirus pandemic.
"We are pregnant and we are so excited to share it with everyone. Baby #3 Thank you @clearblue #clearbluepartner #clearblueconfirmed.
"During this unprecedented time, we know pregnant women may be under greater stress, which is why we are supporting @marchofdimes Mom and Baby #COVID19 Intervention and Support Fund. Check out their Instagram page to see how they are helping moms and babies get the care they need now and in the future."
