/ip firewall nat
add chain=srcnat action=masquerade out-interface=testvpn

/ip firewall mangle
add chain=prerouting action=mark-routing new-routing-mark=testvpn src-address=192.168.3.0/24

/ip route
add dst-address=0.0.0.0/0 gateway=testvpn routing-mark=testvpn