OK, i got a little confused after rereading this article.
http://www.pokerstrategy.com/strategy/no-limit/993/
The table shows that we have from flop to turn 4:1 on flush draw and 2:1 from flop to river.
For simplicity lets assume we have a flush draw on the flop and got offered 3:1 odds. Can we call? I always thought we are drawing 2 cards, so in that situation we could call it. On the other hand from the article I understood that we are drawing on one card from flop to turn if we are not going all in on flop which would let us see both cards.
well, lets take out the rest stuff like implied odds and so one. Just pure mathematics. Can I call flush draw on flop offered 3:1?