DNS load balancing is the practice of configuring a domain in the Domain Name System (DNS) such that client requests to the domain are distributed across a group of server machines. A domain can correspond to a website, a mail system, a print server, or another service that is made accessible via the Internet.
In your case, the group of servers are spread across cloud providers. Lots of caveats and challenges, but it is possible.
You can take this idea is use GeoDNS with health checks.Traffic would be split based on geographic regions, which wouldn't necessarily be even, but the health checks could help redirect traffic if one cloud went down.