1. Friday the 13th is believed to be an unlucky day.

Question

2. Thirteen is often considered an unlucky number.

3. Thirteen is often considered as an unlucky number.

[Are they all grammatical? What about #2 and #3? Do we have to put 'as' after 'considered'?]

Answer 1

"as" is unnecessary, but it's okay. They are all grammatical.