The proposal from @Darius is one variant.
The other variant is to use a static route in a new routing table towards the ISP (taking in consideration that the default gateway is the VPN) and 3 rules (each for every wireless device) to use the new routing table.
Change the src to the IP of the wireless hosts. If they are not static, you can make some static leases in DHCP.
Then use the appropriate interface instead of vpn, the default is wan.