I have a redis cluster with 6 nodes {3 Master and 3 Replicas running in Kubernetes}, will the replicas take-over the master nodes if I delete all the 3 master nodes simultaneously.
NO, these replicas will fail to take-over.
In order to successfully failover, you must ensure there're N / 2 + 1 masters alive. In your case, you must ensure that 2 masters are alive, so that a replica can be elected as the new master.
Only master nodes can votes for promoting a replica node. If all 3 masters are killed, no replica can be promoted.