If you feel comfortable with C++, know how to create a window and manage its input and how to draw 2D elements and the basics of how 2D works.
Then go for it!
When making the transition to 3D i will highly recommend (its a must really) to readup on some math. Not just any math, linear algebra.
The topics to focus on will be vectors and matrices to begin with.
I cant stress the importance of knowing how these mathematical concept work.
Now if your not that much into math (like my self), dont be afraid.
You dont have to write all the mathematical functions yourself, DirectX will provide you with all that you need.
But, in order to write a 3D application one really need to understand how to use these mathematical functions.