Your company has an on-premises network and three Azure subscriptions (Subscription1, Subscription2, Subscription3). All resources are deployed only in West US or West US 2. You plan to connect all three subscriptions to the on-premises network using ExpressRoute. What is the minimum number of ExpressRoute circuits required?

You only need one ExpressRoute circuit. An ExpressRoute circuit connects your on-premises network to Microsoft's global network. Once established, this single circuit can be connected to multiple Azure Virtual Networks (VNets) across different subscriptions within the same geopolitical region. Since all resources are in West US or West US 2 (which are in the same geopolitical region), a single circuit is sufficient. You would then use ExpressRoute Global Reach or VNet peering to connect VNets in different regions if needed, but that's not the case here. Two or more circuits would be redundant and unnecessary for this scenario, incurring higher costs without additional benefit.