The German rules say:
Besitzt der Spieler keine ausgespielten Epochenkarten oder nur welche, die bereits gedreht wurden oder weniger als 3 Stärke aufweisen, darf er keinen Konflikt austragen.
in English roughly (sorry if the translation is not that good):
If the player has no Era cards in play or only ones that are already turned or show less than 3 strength, he may not enter a conflict.
The English rules say:
If you do not have an Era cards in your display that have not yet been turned, or if you do not have a higher Strength than your opponent, or if you do not have at least 3 Strength, then you may not enter into a conflict.
Beside the typo of "
an Era cards", the part "at least 3 strength" is not related to your (unturned) Era cards in play but to your current total strength.
I assume that the English rules are correct and the German rules are wrong and in addition not clear (printed value? what about e.g. leader cards? Camouflage? ...)
Perhaps Bernd could clarify this.