I've got three sites with same IP hosted at the same server, if I put a link from one of them to another, will this link be considered as an external one, or it'll be considered as an internal? Normally it has to be external but have some doubts
As long as they dont have the same or duplicate content you will be fine. We host over 100 sites on our dedicated server that point to each other and all have High Page Rank not effected by the same IP Address.
Cross linking between site like this will not be liked by Google. If it is done to excess (all pages link to all pages), Google will apply a penality. jvfConsulting has been lucky so far. Hope no one rats them out for it.
Since the sites are separate domains they are considered external links. If the sites are related you won't have any problems with google by interlinking them. Don't do site wide links between them, instead pick a few pages that are highly related and link from there.
there is no problem if you have 2 sites only the problem is if you have a dozen of sites linking to each other having same IP this is flagged as Link Farm and penalized by Goooooogle
well some search engines will not consider it as an inbound link. you will have not benefit of getting links back from sites hosted at same IP and same Hosting.