My 3.5m doesn't wait to open mouth wide and wants to feed immediately else starts wailing loudly. How to get a good deep latch?

Once the baby latches you can use your fingers to widen his latch. Apply gentle pressure on the chin to pull it wider. All this while make sure baby is tucked close to you so that once you have opened the jaw you can pull baby close and achieve a deep latch. Having said that if you feel that the baby is able to feed is gaining weight and you are not experiencing any discomfort then the baby is latching well and you need not disturb his method.