Hello @KiptonM
Thanks for your question,
In fact, the PD8 is only available on LQFP144 , UFBGA144 and LQFP100 packages as mentioned in the DataSheet, for that, the STM32F446RE which has the LQFP64 package, the USART3_TX signal cannot be assigned to PD8 pin.
Now, if you want to use the PC10 as pin for the USART3_TX signal, you have just to click on PC10 and choose the USART3_TX (before the activation of the USART IP or even after).
I would like to take this opportunity to present you another way to know the possible Pins that can be remapped instead of other pins for a signal used, without referring to DS and directly through the Pinout view of the CubeMX interface, you need just to put the cursor on pin (which has already a signal on it) and press CTRL+Click, then CubeMX will give you all possible pins which contain the same signal (Alternates) :
Hope my answer helped you, when your question is answered, please close this topic by choosing Select as Best. This will help other users find that answer faster.
Thanks for your contribution, Khouloud