My apologies, when I looked at the documentation it showed IP addresses for the internal server URLs. You may be able to get away with
http://servername internally or inside a DMZ that has the server name exposed, although in my testing and site setups, I haven't been able to. **note they were all v7 instances within a DMZ, v8 may be different as well as an intranet site.
You stated you would have three different sites so why would you want to have an alias for different domains? If you're talking about adding them for the scheduled, tasks, that should only need to be added to one site so it is at least accessible. I'm not stating they won't work, I'm stating in my experience I was able to eliminate errors/issues with a web farm environment by adding the aliases. It could have very well been the environments I was working in were not setup right from the load balancer but I know what worked in several instances and what didn't.
As I mentioned you can have multiple websites on a single instance of Kentico within a web farm environment. It does make it easier to have the sites separated and setup, although maintenance is and will continue to be a huge nightmare. You'll find yourself making the same changes across 3 instances multiple times not to mention the hotfixes/upgrades you've already mentioned. Also you won't get the benefit of sharing any of the work you've done in one site with another site (shared layouts, webparts, templates, etc.)