Kobe Bryant -- FOR THREEEEEEE ... Vanessa Preggo With 3rd Kid

Now we know how Kobe Bryant's been keeping himself busy in retirement ...

Vanessa Bryant just announced she's with child -- the couple's 3rd kid -- and it's a girl!

Mrs. Mamba broke the news on social media -- saying, "Beyond blessed and excited to share that we are expecting our third baby girl!!! #Blessed #BabyMamba #Thankful."

It's been a while since Kobe's changed a diaper -- his oldest daughter Natalia is 13 and Gianna is 10.

Congrats!