I believe you meant 'on-premise' by saying 'main sites'. If so you can link the on-premise vCenters in Linked mode to the VMC vCenter with the feature 'Hybrid Linked mode'. Please refer the following article for more details.
If you have two OnPrem vCenter's linked by ELM and now you wish to include VMC vCenter , this is possible
From the VMC vCenter >> menu >> administration >> Linked Domains ; here you need to setup connection with any one OnPrem vCenter