You'd need a Thunderbolt 3 to Thunderbolt 2 adaptor, and then a mini display port to display port cable, or if such a thing exists a thunderbolt 3/USB-C to display port cable.
doesn't work. TB2 adapter doesn't support miniDP. My 4K Dell is worthless without the $70 Apple HDMI adapter. The only thing the TB2 adapter is good for is the TB display.