Hi,
The solution is not straight forward. You can use 2 MX firewalls (in separate network), would also require separate licenses, not HA. Terminate four WAN links on these 2 MXs. Now, you would also need to do PBR on the Core SW to send traffic to either MX1 or MX2.
Another way to achieve 3 WAN links is to connect 2 WAN links on MX1, MX1 Lan connects to WAN1 of MX2. MX2 WAN2 port connects to Internet link. MX2 LAN connects to the Core Switch.
Both solutions are not that great, simple and not cost effective, but if you really need WAN links more than 2.