I have 4 interfaces for LAN, WAN1, WAN2 and WAN3 and want to set PPTP Server for WAN1, WAN2 and WAN3. If I set gateway to WAN1, I can sucessfully VPN to WAN1 but not WAN2 and WAN3. I tried setting gateway to all WANs but it does not work. Anyone has tried this config, please help. Thanks in advance.

as far as i know it's not going to work...
because... your pptp response can go out from a different network card...
you probaly did some routing loadbalancing...
you must keep the pptp traffic from one interface on that one... and don't balance it...
no idea how to do that...
but...

if it's for static pptp connections...
you could make a mangle rule to mangle all traffic coming from one ip to use a specific routing chain....

this way you can make 3 static pptp session on different interfaces...