First let’s consider blow 3. Your blow notes are arranged uniformly in terms of pitch. If you blow them in sequence from 1 to 10 you will hear the main notes of the C major chord in arpeggio. (What’s arpeggio? Blow 1-2-3-4-3-2-1. Sound a bit like an opera singer warming up? The same happens with holes 4 to 7 and holes 7 to 10, except each time you are moving up an octave…erm it’s the same arrangement as 1 to 4 but the notes sound higher).
The principle notes of the C major chord are C E G C. Blow 3 is G. Without this note the chord would be incomplete.
Now let’s consider draw 2. Your draw notes are not arranged uniformly in terms of pitch. This is deliberate so don’t worry. (It enables the player to reproduce melodies around the central part of the harmonica, in the key of the harmonica…which is what it was originally designed to do). Draw holes 1 to 4 in sequence up and down. Not exactly arpeggio, but not far off. What you have are the principle notes of the G major chord.
The principle notes of G major chord are G B D G. Draw 2 is G. Without this note the chord would be incomplete. In fact we would have to call it something else entirely!
So G natural is an integral feature of the C major chord and, of course the G major chord. You therefore need one when you blow and one when you draw… I have yet to meet someone who can do both at the same time.
A classic question for beginners is whether to finish a lick on draw 2 or blow 3. My answer is it depends on the lick. There are always exceptions, but the basic rule is… when playing cross harp, draw 2 should be your destination. When playing straight harp, blow 3 should be your destination. To put is another way, on a C major harp, if you are playing over the G major chord, use draw 2, and if you are playing over the C major chord, use blow 3. It’s why they are there. I repeat there are always exceptions, however longterm this is correct musically.
Supplementary tip – when starting out, 2 draw can be hard to master. You can always blow 3 to see if you are in pitch. (See ‘Why is 2 draw so difficult?‘).