The best way to win as France is to mass the northern and southern armies in central France in one of your vassals, then wait for either A) the English armies to unite and die of attrition, and/or B) Hit them when they split up with your combined army. If you're lucky, you should win a battle or two this way, and follow up by pursuing the English army. You might be able to cut the English in half this way, which makes things much easier. 