NS1 is a bit different than traditional DNS servers and services in that an "A record" in NS1 is a collection of answers. The real power of NS1 is in the granular control the rest of the Filter chain offers you over your DNS records.
Instead of configuring several records for the same domain, configure a single record and add multiple answers to the record. To implement a basic round-robin behavior, you can simply add the Shuffle filter to your filter chain. Use the Analyze Filters tool to see how the filter will work in real-time.