Bescheidenheit im
Einsatz der Mittel
Beschränkung
auf wenige Ideen
Zu viele Effekte
nehmen sich gegen-
seitig die Wirkung
More than two fonts cause restlessness. Also the construction as regards content cannot be shown logically and sensible anymore.
Anyway you shouldn't mix similar fonts (tell me why?). Better mix a serif-type (roman) and a font without serifs. Within a type-family you can mix without danger. (Watch out: Typomanics!).
In this example (left) you can see well that different type sizes in the same document have an unfavourable effect. So you should confine yourself to two or at most three sizes and use this means only for structuring.