You're not going to find a Thunderbolt 3 to Thunderbolt 2 cable as an adapter is required to connect to legacy devices.
One of the (major) differentiators between Thunderbolt 1/2 and Thunderbolt 3 is that Thunderbolt 1/2 required the cable to be active (it required power to operate) whereas Thunderbolt 3 can use standard USB Type C cables (it's passive)1.
With these older Thunderbolt standards, the cable was active, meaning
the cable itself is a device that requires power to operate (which is
why most Thunderbolt 1 or 2 devices would require an external power
source in order to function.)
Thunderbolt 3 is backward compatible, but will require an adapter.2
From the FAQ:
Is Thunderbolt 3 backwards compatible with Thunderbolt and Thunderbolt 2?
Yes, solutions and products built to Thunderbolt and Thunderbolt 2 specifications will work with Thunderbolt 3 via an adapter.
To make your setup work, something like the StarTech TBT3TBTADAP TB 3 to Legacy TB adapter.
I've personally used this adapter to connect a 2018 MacBook Pro to a legacy (Thunderbolt 1) WD MyBook Duo and it worked with no issues.
This kind of adapter is only compatible with Thunderbolt devices, not Mini DisplayPort.
1 C|Net: USB Type-C and Thunderbolt 3: One port to connect them all
2 Thunderbolt Technology Frequently Asked Questions