Weakspots

pizzastorm: IS-6 - Can pen the square hatch on the front most likely if the tank is facing you directly. If it is angled then it will be hard to pen almost everywhere. You can also go for the hatch at the top although it has a low profile
Is-3 - Either side of the pike that is turned towards you. Lower part if turned directly at you.
Type 59- Most of the lower hull you should be able to pen.