I am a new comer in UK I don't understand why BS 1363-3 multiway adaptor not allow to use. is it illegal to use or not encourage to use?

I am not aware of law of the land that prohibits the use of these mutiway adaptors.

Many however regard use of these are being poor practice, especialy the unfused ones.
Many employers and workplaces prohibit them, and one should abide by any such rules.

Some "jobsworths" claim that anything prohibited by their or their employers policies is "illegall"

I see no harm in the use of multi way adaptors by sensible persons, for low loading appliances.
Use for heavy loading appliances is unwise and potentialy dangerous, and since common sense is in fact uncommon, many employers ban them.

If it's a genuine BS1363/3 multiway it's unlikely to be illegal. A lot of institutions don't like them though, I think the safety guys worry that allowing them is encouraging overload - they are usually shown photos of misuse like the one below as part of their training. They also tend to break the socket when misused and sometimes they get crowded and you can't plug appliances in fully.

Where I work we allow multiway extension leads (socket boards) all over the place when sometimes a BS1363/3 block adaptor would be more appropriate. As an institutional PAT tester I pass these as safe so long as they're being used properly, I think at one time we had a ban on them though.

http://www.sciencephoto.com/media/343176/enlarge