WAN custom routing


Hi All,
I have a little strange requirement, hoping to find a solution in this forum.
Customer has three ISP connections ISP1= 75 Mbps (on WAN1), ISP2=75 Mbps (on WAN2), ISP3=150 Mbps (on WAN3). Under normal condition, outbound traffic should be load balanced over WAN1 and WAN2 only with ratio of 50/50. WAN3 in this case should be standby. Should any one of WAN1 or WAN2 go disconnected, all traffic should be routed over WAN3 only.

W1= UP, W2= UP —> W3=STD-BY, LOAD BALANCE 50/50 over W1 & W2

W1= DWN, W2= UP —> W3=UP, TRAFFIC enforced over W3

W1= UP, W2= DWN —> W3=UP, TRAFFIC enforced over W3

I have thought to achieve this in the following manner, for which I need confirmation if it will work (or maybe suggest a better idea):
1- WAN1 will be “always on”, health check= PING, PING target= W2 IP address.
2- WAN2 will be “always on”, health check= PING, PING target= W1 IP address.
3- WAN3 will be “Backup”, health check= DNS, DNS Host target= Public DNS, ISP DNS.

Outbound Rule1: Source: any, Destination= any, algorithm: load balance: W1=5, W2=5, W3=5

Appreciate your comments and feedback