A3 on the keyboard
A3 is one key on a keyboard.
A3 is one key on the keyboard, and no other key sounds that pitch. The coloured keys are the other As, each in a different octave.
A3 on the neck
A3 lies at string 6 fret 17, string 5 fret 12, string 4 fret 7 and string 3 fret 2 on guitar — standard (eadgbe).
The same pitch can be played in four places: string 6 fret 17, string 5 fret 12, string 4 fret 7 and string 3 fret 2. Each one sounds identical, but each feels different under your hand. The faint notes are the other As, in higher or lower octaves.
A piano has one key for each pitch. A guitar does not. Most notes in the middle of its range can be played in three, four or five places, and although they are identical in pitch, each one feels completely different under your hand.
This is the real reason the neck feels confusing at first. You are not learning more notes than a pianist. You are learning the same twelve notes several times over, in different places.
It is also why playing in positions works. When you choose a box, meaning a group of notes that fits under one hand, you are choosing which copy of each note to reach for.