Planning for multi-cloud backup tends to be tricky, because every environment involving multiple clouds is different. As you develop an approach for protecting a multi-cloud environment, you must consider which clouds you are using, what services you are running within each cloud and what your data protection goals are for each the cloud-based resources.
One the most long-standing best practices for on-premises data protection is the 3-2-1 rule. That rule essentially states you should have at least three copies your data, on two different types backup media, with at least one backup copy residing off-site. Unfortunately, this rule can't be directly applied to multi-cloud backup -- we'll detail the reason later -- but you can still adhere to the spirit of the rule.
The main idea behind the 3-2-1 rule is you don't ever want to find yourself in a situation in which your backup can potentially become a single point of failure. The requirements for multiple backup copies and off-site backup storage are meant to guard against the possibility that your one and only backup becomes damaged.
In a multi-cloud backup environment, I recommend storing a copy on the same cloud where your primary data resides but also replicating that backup to the other clouds you use. If a data recovery operation becomes necessary, you would ideally want to restore the data from the backup in the same cloud as where the primary data had been kept. After all, it is going to be much quicker to restore a local cloud backup than to have to transfer backup data across the WAN link that exists between clouds. At the same time, though, having backup copies stored on other clouds can protect you against a cloud-level disaster.
Part of the 3-2-1 rule is the idea that your backup should be stored on at least two different types of media. That part of the rule doesn't really work in a multi-cloud backup environment. What you can do instead, however, is to take advantage of cloud regions. If the bulk of your primary data is stored in a U.S. cloud data center on the West Coast, then you might consider using an East Coast -- or even a European -- data center to store one of your backup copies.