From what I think you are asking there shouldn't be any difference irregardless of whether you register the domains through eUKhost or 1&1
as long as everything gets configured correctly!! (such as nameservers etc). So you will be able to have 3 different webspaces (as long as you get the relevant hosting packages) and you will be able to have email for all three domains, again subject to the hosting package you pick.
Finally, as far as I am aware, the browser / surfer won't notice any difference at all because it doesn't make a difference to them, visible or otherwise, who you register the domain with.
Hope that helps
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