@Vicky Thank you for reaching out to Microsoft Q&A.
I understand that you want to know the possibilty of connecting Multiple VNets belonging to either same or different subscriptions to Azure Express Route.
All the three scenarios that you described are possible with Express Route. However, it requires that the circuit owner authorizes the administrator of the other subscription to use the specified circuit. Please refer to this document for more details.
Hope this helps. Please let us know if you have any further questions/concerns. Thank you!
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