I used netselect-apt package to find out the fastest servers. Can I enter more than one mirror in sources.list so that if one mirror is not reachable, next will be used automatically. eg:

Yes, that is totally possible and works.

You may be interested in using a redirector, more details can be found here.

e.g.

Thanks for the information. To use the redirector, I think I need to put in following code in sources.list:
Currently, I only have last 3 lines in sources.list. I retain those lines so that they are used if the redirector does not work. Am I correct?

Can't say that you are wrong for being careful.

I do have a habit of looking at outputs of commands used to ensure that there are no errors reported.
In this case, aptitude or apt-get update will give very obvious error message if mirrors are not available.

In the very rare case that redirector does not gives me a mirror that works properly, I'll manually update the source list to point to a specific mirror and do an update again.

I'll switch the source list back to redirector after that, since it works 99%(?) of the time. A little more work but worth it nevertheless.