Hi, I was wondering if I Setup 3 Node K3S Cluster managed by my rancher-manager, if i will need as well a mysql as suggested here https://ranchermanager.docs.rancher.com/how-to-guides/new-user-guides/infrastructure-setup/ha-k3s-kubernetes-cluster or the mysql would be just necessary if k3s is being used standalone, without being managed by rancher-manager?

no. a 'regular' 3 nodes clusters should use etcd

thank you @handsome-elephant-92657.. so just follow the https://docs.k3s.io/datastore/ha-embedded to deploy the K3s cluster and import it on rancher manager ?

If you are trying to create a cluster from Rancher UI then simply click create button follow the prompts and copy/paste the registration URL.