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Yes, you can use your L3 switch to provide the router a "3rd" port. The trick is, you'll trunk from the router to the switch. This will allow you to have multiple logical (sub interface) interfaces on the trunk port.
The switch doesn't route to the ISP, it just provides an access port, whose VLAN is passed on the trunk to the router.
If any of your switches are stacked, you can also run your two router ports to a different stack members, forming a MEC, and trunk both your inside and outside logical ports on that MEC, also configured as trunk. Then no one switch, or one ISP, or one port on the router is a single point of failure.
