I suppose he has 3 internet connections to the same ISP and in order to get WAN access he needs to dial in via PPTP but has to make sure that each PPTP client uses a specified connection.
unfortunately, it’s not possible in current version
you may write to support and ask for a possibility to select source address of pptp client (like Update Source in BGP Peers, etc). then you will be able to use Policy Routing to select appropriate route based on source address
for now, you should use three different routers (KVM virtualization? )