So these are autotransformers 220kV to 33kV, with a delta tertiary at 11kV for house/station power?

Since in an autotransformer the primary and secondary are really just the same winding with a tap, the neutral bushing is really common to both. On single-phase units, that means there is an H bushing, and X bushing, and there only needs to be one neutral bushing that is shared by both H and X, usually named H0X0. I suppose the mfr may provide separate H0 and X0 neutral bushings outside for your system connections, but internally they are connected to the same point in the winding.

The 3 neutral bushings of the 3 in-service transformers should be connected to the neutral pipe bus individually. That forms your Wye connection. If there are separate external H0 and X0 bushings, then you have wasted your money on extras that are not needed. Just use the H0 bushings to connect to the pipe bus, and ignore the X0 bushings. Your primary and secondary system neutral conductors will also connect to the pipe bus, not the bushings directly. Then there should be ONE connection from the neutral pipe bus to ground.

Outside, unbalance currents are going to sum in the neutral conductor. When you connect the neutral bushings of the transformers to the pipe bus, any unbalance currents in the outside system will divide themselves up to the appropriate phase with which they are associated. There should be no unbalance current in the ground conductor! The only time you should really have any current in the ground is when there is a ground fault, and your protection should detect and respond to that.

Thanks a lot for the reply. But they are not auto transformer they are they are three winding transformer. Each transformer is having six bushing (HV line, HV Neutral, MV Line, MV Neutral, and LV1 and LV2 with HV=220kV MV=33kV and LV=11kV). So there is internally separate neutral for 220 and 33kV. If you can share your mail id I can mail you the layout and one line diagram of substation.

What is the specified connection of the secondary? Grounded wye, or delta? If wye, then the three MV neutral bushings go to a secondary neutral bus that gets grounded. The HV neutral bushings have their own neutral bus that gets grounded (separately). It's just as if it were a grounded wye-wye, 3-phase transformer: The primary wye's neutral gets a connection to ground, and the secondary wye's neutral gets a connection to ground. Just draw the 3-line diagram and you will see.

Any specif reason for doing this separately? that was my main intent of asking this question. What will be overall impact of doing it with common neutral bus for both the windings?

I have seen wye-wye transformers with a common, internal neutral that has just one neutral bushing (H0X0) coming out to the external ground connection, so there is no reason physically that it couldn't be done as one external neutral bus. You would have to talk to whoever is doing the relay protection design to see if there is any reason there for having the neutrals separate, but I don't think there is.