[ok, may be a stupid question, but I can't ever seem to get more than three connections.]

As long as you mean '3 connections to ultrapeers' this would be the same here. I have wondered for myself, though.

[I was told by someone that you must have five connections, in order to be able to download items - but as I have said I can only ever get three?]
Definitely not true. In the extremest case even one connection would be enough to find some files and download them.

[but always get the message "waiting more hosts needed" when I try to download something]
Well, you will get this message when the server you are trying to download from does not respond in time; the most common cause for this is (I think) when the server is firewalled (push request) and does not 'push' in time.

Sadly, this is a problem you cannot mend, as there are many people out there giving either a damn about sharing or are technically not able to configure their firewall right.

Greetings...