One for All, and All for One

Question

We three are one
For ear or tongue
Yet not for eye nor hand
Of our good three
But one lacks three
As this riddle lacks we

Answer 1

These are

the words to, two and too.

We three are one
For ear or tongue
Yet not for eye nor hand

They sound the same, but are written differently.

Of our good three
But one lacks three

Only one of the words has fewer than three letters, the others have exactly three.

As this riddle lacks we

None of these words appear in the riddle.

Note that

There are other possibilities, eg. so, sew and sow, but given the references to one and three, two seems the most natural answer.