Correct Answer: To deploy an Azure Firewall in an existing virtual network VNET3, follow these steps. First, create a special subnet named AzureFirewallSubnet (required for Azure Firewall deployment). Go to Azure portal, search for Virtual Networks and select VNET3. In the overview section, take note of the region and resource group. Navigate to Subnets and click on + Subnet to add a new subnet. Name it AzureFirewallSubnet and specify an appropriate address range. Click OK to create the subnet. Next, add the Azure Firewall by going to the Firewall option in VNET3 settings. Choose `Click here to add a new firewall`, ensure the resource group and region match VNET3 settings, enter a name for the firewall, and associate a public IP address. Click Review + create and then Create to complete the deployment. Ensure the new subnet has a /26 size for optimal functionality as recommended in Azure documentation.