Hello
Your requirement is not exactly clear. If you want to plan all the plants together, you should use a scope of planning when running MRP.
If you want to see the MRP results together, then you should activate the cross-plant option on MD04. See blog Material Grouping and Cross-Plant View in the Stock/Requirement List(T-code MD04).
If you want to have MRP planning stock transfers from one plant to another, then you should use a special procurement key on the material master.
If you want to have MRP considering the stock of several plants together, you will find several threads where it was suggested the utilization of a BAdI for such purpose. See thread MRP Run considering Stocks for Multiple Plants ... | SCN.
BR
Caetano