customized results requires customized solution
Stop using netwatch.
instead go with script method. it has utmost power and you can get about any result you want.
If you have 3 wan links, then you should have at least 2 monitoring host (e.g 8.8.8.8 or 1.2.3.4 likewise) per wan to monitor.
means create static route for each monitoring host going via specific wan link. also its better to create Blackhole route for them as well to make sure no matter what happen ping to that host goes via that link only.
Following is just an example for you , just to help you with designing your own one !
https://aacable.wordpress.com/2015/09/1 ... -approach/