If two eighth notes make up a beat, how many sixteenth notes would make up a beat?

4.
Since you are multiplying 8 by 2 to get 16, you would multiply 2 by 2

It depends on what type of third separates the three notes.for example: if you have two mayor thirds is an "aumented chord"if you have two minor thirds is an "diminued chord"if you have one mayor first and then one minor is a "mayor chord"and inverted is a "minor chord"