Information on the OSSM v3 docs doesn't have any information about how to get a must-gather like v2 had and in the OCP documentation regarding must-gather images, the new v3 image is missing, since the one referenced is only valid for OSSM v2.

https://docs.redhat.com/en/documentation/openshift_container_platform/4.18/html/support/gathering-cluster-data#gathering-data-specific-features_gathering-cluster-data:~:text=Table%C2%A05.2.%C2%A0Supported%20must%2Dgather%20images

You should reference both since for the OSSM v3 the new image is:

registry.redhat.io/openshift-service-mesh/istio-must-gather-rhel9:3.0