According to the Microsoft documentation, Azure Bastion is a service that provides more secure and seamless RDP and SSH access to virtual machines without any exposure through public IP addresses. You can provision the service directly in your local or peered virtual network to get support for all the VMs within it.
In your scenario, you have three virtual networks that are peered with each other. This means that they can communicate with each other as if they were in the same virtual network. Therefore, you can deploy one Bastion host in any of the virtual networks and use it to connect to all the virtual machines in the peered virtual networks. You don't need to deploy a separate Bastion host for each virtual network or each virtual machine.
For more information about how to deploy and use Azure Bastion, see Tutorial: Deploy Bastion using specified settings: Azure portal.