@VITSUSAApr 20.2018 — #Sub-Domain (www3) is used to load e.g. images from another Server - so Domain is equal but Sub-Domain is different means that can be different server machines e.g. for loadbalancing etc.
@efusionworldApr 21.2018 — #WWW2 and WWW3 are hostnames or subdomains, typically used to identify a series of closely related websites within a domain, such as www.example.org, www2.example.org, and www3.example.org; the series may be continued with additional numbers: WWW4, WWW5, WWW6 etc.
@learncoadingApr 22.2018 — #w3- stands for the world wide web consortium, they are the ones who started the development of the world wide web. As a visitor, you should not worry about it. www3 it's the same as another subdomain names, e.g http://subdomain.domain.tld. If you are a developer you may manage traffic to different subdomains to balance channels consumption.