How do I play this tab ?
I confused since no fret number is written on the 3rd string, and I guess it's not an open string either. Do I pick those two (2 and 4) strings only?
What I did is just to strum those 3 strings with the middle one as open.
Doing what you say you did sounds pretty awful, as the F#s clash with the open G. If the writer wanted you to play G open, he would have put 'o' on that string. Otherwise someone could play all 6 strings, 4 open and the other 2 as written. So play the 2 F#s, either with finger and thumb, or with a pick, muting the G (3rd) string in between as you strum. You can mute the 3rd string with either the tip of the finger on the 2nd string, or the flesh of the finger on the 4th string. Some tab either side of the posted may put this into a better perspective to analyse.