How much time before my visa (3 years visa as married with a German National) expires? Can I apply for a new one?
First of all: you have a residence permit (visa is only issued by counslates and then only for a short of about 3-6 months).
On your question: check out the information of your responsible immigration office.
Often they give advice as to when to hand in application for extensions.
In Berlin, a new application can only be applied for 8 weeks before the current permit expires.
An appointment, well in advance, should therefore be made between the 7th and 8th week.