I understand that for 3Essentials to host my DNS, I should point my domain registration (i.e., delegate your domain) to 3Essentials nameservers:

ns1.3essentials.com
ns2.3essentials.com

However, I would like to "brand" these under my own name, so that my clients don't see the 3Essentials name.  Is that possible?

Yes.  You would simply create "A" DNS records on your domain that have your branded DNS names resolve to the IP addresses of our public DNS servers.  For example, if your domain was myfancyhosting.com you would create A records like this:

ns1.myfancyhosting.com A 173.227.253.196
ns2.myfancyhosting.com A 72.29.101.108

And then you would create NS records for your domain that point to those A records:

ns1.myfancyhosting.com NS ns1.myfancyhosting.com
ns2.myfancyhosting.com NS ns2.myfancyhosting.com