It depends on the context. Literally: "la France dans 3 ans" Mais si "I will visit whole France in 3 years" > Je vais visiter toute la France en 3 ans de temps/dans une période de 3 années

Just to expand on Olivier's comment, "in" has many meaning. It's what I call a can-of-worm word. Do you mean you will go in France in 3 years time? it would be "dans 3 ans". Do you mean you visited France in a 3 years round trip? it would be "pendant 3 ans" or "en 3 ans".