@CJR_ZA if you want more than two WANs then you need to use the make a wish feature in the dashboard, there is a button labelled as such at the bottom of the screen.
To create the DMZ enable VLANs on the MX and then create a second VLAN with an interface IP and apply it to a port. You can then create separate rules between the LAN, WAN and DMZ
If my answer solves your problem please click Accept as Solution so others can benefit from it.