Calendar synchronization issues on ipad

Assuming that your Mac, iPhone and iPad are all signed-in with the same AppleID as stated, have you enabled the required iCloud services on the iPad?

Settings > [Your Name / AppleID] > iCloud - enable all the Apps and Apple services that you wish to synchronise

Perhaps counterintuitive, for each of the enabled iCloud Apps and services, switch each to OFF on the iPad. Having switched them all to OFF, exit the settings App - and then force-restart the iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

After restarting the iPad, return to your iPad’s iCloud settings - and again turn ON all Apps and services that you wish to synchronise.