Is it possible for me to travel in the entire network territory with my ZVV travelcard for 3 zones and a 24h-ticket for 5 zones? As I have paid the price for 8 zones.

No, you can travel in the entire network territory only if you purchase a zone upgrade for the missing 5 zones in addition to your travelcard. You require only one zone upgrade for 4 zones to travel in all zones in the case of the monthly 9 O'Clock pass or the annual pass of the City/Agglo Zurich or the City/Agglo of Winterthur as well as for the ZurichCARD.