Question: I have recorded a vocal for my music.  

Now I want to pitch my vocal, as to also have a 3rd harmonizing vocal. I have heard a lot that people say that a 3rd voice is the best voice and sweetest one for making a harmonized vocal. But I wonder if they are talking about a minor 3rd, or a major 3rd voice? 

I read that if you play C and sing it, then your 3rd voice will be E. But what if I play F sharp, should I harmonize it with A sharp or with A?  

(I know that I could sing any note for harmonizing, but I just wonder which one is for 3rd voice. )

Answer: 
The basic harmony is usually a third above (or a 6th below, which is the same note an octave lower. Example: **in C** - sing a C - the harmony will be an E, either the one directly above, or directly below. So here, we can call it a major third above, for simplicity. However - in key C major, the note that harmonises with D will be an F. this is because they're *both* from the C key - diatonic. Technically, it makes *that* interval a minor 3rd, but so what? It sounds good, and doesn't suddenly make the song go into a minor key!

When you need a harmony for an F note (still in key C), it'll be an A - straight from the C scale, as that A is a third above the original F.

'When someone sings an F diez(#)' it will depend on **which key you're in at that time**. If it's the same song, in C, then I'm guessing the underlying chord is a D major/seventh, and thus an A natural will fit best.

To summarise - use notes usually from the key/scale at the time. Use notes from the harmony used at the time, which may be different, but usually still diatonic.

If you're just singing along, really, you shouldn't worry about whether it's a major or a minor third you're actually singing - your ear ought to tell you. Maj/min 3rd is more of academic interest - and a heck of a lot of folk who harmonise spontaneously won't even be aware what notes they are singing - except that they'll be the right ones, judging by the best judges we use - our ears.
It's good to HARMONIZE, not just slavishly double the melody a fixed interval above. Yes, thirds are useful. Sometimes they fit above the melody, sometimes under it. This will very likely change over the course of the song. Sometimes it will be best to sing a few notes in unison, then open out into harmony. Sometimes one singer can echo a phrase that the other has just sung. Another good trick is to find the dominant (5th note of the scale) and stick on it until your ears tell you it MUST move one note up or down.