4.Can I replace the gimbal of the Osmo Mobile with a Zenmuse X3?
No. The Osmo Mobile is sold as a complete unit. The gimbal is therefore non-detachable.
There are two models of Osmo Mobile: Osmo Mobile (complete unit) and M1+ handle, if you are referring to the first one, the gimbal cannot be detached for another gimbal, sorry for the inconvenience.