For example I have 3 PCs and I set them to get the work from public pool. I have to create 3 different worker username (e.g. user.1, user.2, user.3). It must be exhausting to setup 100 workers because beside I have to create 100 different username, I have to create 100 different script too :(. Can I have my workers to use one worker username (e.g. only user.1)? Thanks
It depends on the pool you are using.
With my deepbit.net you can use 10 (may be even 20) miners on single worker account with long polling support.
If you don't use long polling then miners number on single worker account is unlimited.