As an heath country you always want to have stability three, boost it by using administration if needed to get there. (more tax money and less risk for rebels)
100 Legitimacy is usually what you start with, and you will lose some when you engage in royal marriage or when the king dies and the heir takes his place. and each of those looses are slowly recovered.
which leave us with Prestige 100, You can use adviser to reduce the decay, but most prestige is gained by winning wars and battles especially against an rival country. bypassing rival countries on the leader board also grant you prestige. Some events will also increase/decrease your prestige.
Eventually all three values will be at the maximum and you will unlock this achievement