Question

How does a third note look like.
What is a unison?

Answer 1

Are you referring to a note with 3 beats?
And unison is when all parts play/sing the melody.

Answer 2

A third note looks like a half note with a dot beside it. It's known as a dotted half note.

Answer 3

and to answer the other question, a unison is when two or more instruments (can be percussion, orchestra, band, a choir, two violinists, etc.) play the exact same thing together (in sync)