There's lots about Just Intonation on the Web. Try The Just Intonation Network for starters. This means integer frequency ratios. Recall that overtone 3 is a fifth above overtone 2. That is, the frequency of the higher note is one and a half times that of the lower note.
Most temperaments keep just octaves, for simplicity if nothing else.
This describes the frequency ratios as well as the overtones. Adding intervals means multiplying frequency ratios. A tone is the difference between a fourth and fifth. But, a major third is two tones. So, there are two different tones in just intonation.