Hello ,
even I am a bit confused here . So just to clear out , let me explain and correct me if I am wrong . You have 3 VLAN Say "A,B,C " and 3 WAN " ISP1 , ISP2, ISP3 " .
You have connected 2 VLANS to trust interface "T1 , T2 ,T3 " and connect 3 WAN interface to W1, W2, W3 .
My setup will be
VR-1 ( T1 and W1 ) interfaces , default gateway will be ISP1
VR-2 ( T2 and W2 ) Interfaces , default gateway will be ISP2
VR-3 ( T3 and W3 ) Interfaces , default gateway will be ISP3 .
Now I see from your update that WAN IPs are behind a NAT box with private IP . I did not get that part , is it that all the 3 WAN IPs point to same private IP or something different .