is this sentence correct?

If you are still studying in this school, than your sentence is absolutely correct.
I have been studying in this school for 3 years.

2009-03-31 11:46:00 補充：
Correction:
than, should be "then".
If you are still studying in this school, then your sentence is absolutely correct.

Yes, you are correct.. Becoz this event lasts for a long period of time (3 years) and it is still happening. So "have been studying" is correct.

"I have been studying in this school for 3 years" is correct.
This sentence is in present perfect continuous tense, active voice.

"Have been studied" is present perfect tense, PASSIVE voice.

The sentence you wrote is correct.
Use "have/ has been (verb)+ing" when you are describing something(s) that have/has started in the past and still currently continuing. Use "have/ has been (verb)in past perfect" only for something(s) that started in the past and have/has been done.

如果你既意思係「我係呢間學校已經讀左3年」，而你仲係呢間學校讀緊，咁你就應該用present perfect continious，所以你既答案係岩既。

We use the Present Perfect Continuous to show that something started in the past and has continued up until now. "For five minutes," "for two weeks," and "since Tuesday" are all durations which can be used with the Present Perfect Continuous.

http://www.englishpage.com/verbpage/verbtenseintro.html
關於tense，呢個網頁幫到你。

應該系have been studied

因為你系話讀左3年

所以應該用過去完成式