multisite single database prefixing

Hi!

Due to hosting costs, I'd like to try having all of my clients three sites fitted in on database. How would I do this?
1. Through prefixing the tables?
Can I specify which tables belongs to which sites in one single database? I'd like to share the user and roles tables though.
Is this the way to achieve it?

Any insights on this would be appreciated! I'm a multisite first-timer, so I'm a bit lost in the worklflow for this..

Thanks!
DrupalNovice