As you are a beginner re-learning the notes, it would probably be a bad sounding F (alto/baritone sax F, concert Ab). If you finger the F as normal (LH123 RH1) then simply press the RH3 down so it is fingered as you are asking, you should hear it sound more muffled. Do you? It is not a standard fingering. (A jazz player might use it as an effect, as sometimes they toggle between two fingerings of the same note so the texture changes, but for now, I would not recommend it as you want to develop clarity in your sound with the correct fingering).

The only other thing I can think of is that one of my clarinets is slightly strange, I can do that fingering (sometimes it is called 'forked') and because of the extra key levers, it is actually a legitimate fingering for this particular instrument (and others made in the same way). Some (vintage, so it could be yours...) saxophones had this also but it would be rare I would think and I have never seen one. If you do have this on yours, it should generate a clear sound a whole step lower than the F—an Eb. Finger a normal Eb then do this fingering and see if you get a clear Eb sound as well. It might sound very slightly different in texture, but should be very near the normal Eb in sound. If you have a friend with a sax you can look at the levers and rods around RH 2 (I presume) and see if there is a difference. (Again, I have not actually seen one to know what to look for, but if you have this, it would presumably be different than a normal sax) and the repairer who serviced your instrument would remember if it has this.

One more thing, very advanced players use different finger and lever combinations to play very high above the normal range of the standard fingered notes (altissimo) but I am not aware of this combination of fingers being used in that way (someone at the forum here might jump in and comment further on this) but I am assuming for your question, it is irrelevant.