what is difference between www vs www1, www2, www3 etc

It doesn’t technically mean anything in particular — domains read right-to-left, so for everything to the left of a company’s domain, it’s up to the company what each bit means. (“www”, for example, is just a conventional subdomain for the web site of the company.)

I‘ve no personal experience of what www1, www2 et al are commonly used for in practice. It might well be different servers, although to my mind that’s exposing implementation details at the interface level, and is thus a bad idea.

This is largely speculative, but generally each of the www'n's is simply a different web server, which a given user has been routed to either manually (for example all images might live on www2, etc.) or by some form of up-stream round robin system or load-balancer, both of which tend to use some component of the end user's IP address or similar to ensure that a user's session will remain on a given server.

Incidentally, more modern implementations will hide the existence of multiple servers behind a single 'www', so that this is less visible/intrusive.