Is there available differential impedance calculation for this stripline configuration?
If your only high speed signals are on layer 3 then you choose stripline.  There is no need stated for an asymetric layer space.
Here is a web tool.

https://www.eeweb.com/tools/symmetric-stripline-impedance

https://www.eeweb.com/tools/edge-coupled-stripline-impedance
https://www.eeweb.com/tools/edge-coupled-microstrip-impedance
https://www.eeweb.com/tools/broadside-coupled-stripline-impedance