Is it true that if you're a good dancer and a singer,means you're good in bed?

To truly have a "good in bed" experience it comes from knowing your partner from the inside out. And having a history with each other. You wont find true "good in bed" in a drunken stupor or a one night stand.

Doing the vertical Mambo good doesn't necessarily convert to a good horizontal Mambo.

Singing? Nah. The last thing I want to do is have somebody break into song when I'm doing my best.

But dancing? Well, yeah. Go read Dark Majinn's excellent answer. This is something American poet Maya Angelou expressed in her poem, "Still I Rise".

Does my sexiness upset you?
Does it come as a surprise
That I dance like I've got diamonds
At the meeting of my thighs?

Well i use to sing pretty good so maybe true

Having rhythm helps, but there is more to it than that. 

That is a good hypothesis. We all know much tales  about the feet size and also the nose. But i have never heard about having the sense of rhythm and good voice may help you to be the better lover.  That is ridiculous.

I've never heard a "Good Singer" is good in bed. I have heard that a "Good Dancer" and I always assumed that it had to do with flexibility ease of movement, and the confidence and sexuality that come from being comfortable enough to release the inhibitions to dance as if your the only one in the room . . . With rhythm.

Nope .. Not even remotely.  All are completely unrelated skills or talents (which ever you prefer to call it)