How to size the Collector Bus?
I have 3 1600kw/400V gengerators Synchronized and connected to the Collector Bus.

Ok I'll bite, (1600 X 125%) + 1600 + 1600 = 5200 amps.
28-108 of the Canadian Electrical code for conductor sizing for Motors.

Method 2 take the connected load at 125% and size the bus for that.

I don't know what a collector bus is but i guessed it is the bus after the transfer switches where each generator parallels.

Yes,Collector bus = paralleling bus.

But,generator is different from motor.

Yes a generator and a motor are not the same but the wires are sized the same. At least that is what i derive from section 28 of the CEC %125 of the first (largest) generator and 100% of the next 2. Generators and motors are very similar in terms of what happens inside them whether making or using electricity. Cooling, winding and loading all apply the same. A stator for a motor and a generator look the same from a winders perspective at least from 28 year old memories they do. I have rewound stators for both. The armatures are different as there is excitation and magnetic fields to build.