Your second statement is right, so saying "it is argued that" expresses the opinion of other people, rather than yourself. It has the same meaning as "people argue that...". If you were referring to yourself and your opinion, you could instead use "I would argue that...". The sentence "it is believed" has a similar meaning to "it is argued", however what someone "argues" tends to have a stronger meaning than what someone "believes" - "arguing" something is more opinionated than just "believing" something. An argument ("argue") tends to be used when people have different opinions, and you are expressing your side of the argument. A belief ("believes") is just what someone thinks or has a personal opinion on, without reference to any other opinions. Finally, "it is said..." refers to what someone has said. Unlike the verb "argue", it tends to have less of an opinion associated with it, it just means what someone has said before. It is the most informal of the three verb choices. I hope I have helped clear that up a little - it is confusing, I know!

No, it does not express your opinion. "It is argued/believe/said that..." simply means "Some people argue/believe/say that..."