USB-C to Macbook Pro (mid 2014) with USB 3 port

First you need to determine what protocol the external hard drive utilizes since a USB-C connector can be used with Thunderbolt or USB protocols.

If you see a lightning bolt symbol next to the USB-C port on the external drive, then it indicates it uses the Thunderbolt protocol.

If you see a trident type symbol next to the USB-C port on the external drive, then it utilizes the USB protocol.

If there is no symbol next to the USB-C port on the external drive, then most likely it supports the USB protocol, but you should check the manufacturer's documentation for the external drive to be certain. Make sure to read the documentation carefully since it may mention both Thunderbolt & USB even though only one is likely to be supported (they like to tell you about USB-C and what it is made to do, but not what the specific product is actually capable of...very misleading with many vendors). If in doubt, post a link to the documentation for the external drive purchased and we can review the manufacturer's documentation to confirm which protocol is supported.

If the external drive supports the USB protocol, then you will need a USB-C to USB-A Adapter cable such as this one:

mophie USB-A Cable with USB-C Connector (1 m) - Apple

If the external drive supports the Thunderbolt protocol, then you can try using this USB-C to Thunderbolt 2 Adapter along with an older style Thunderbolt cable utilizing the mini-display port connector. I've never had any occasion to try this as I've never had an external drive supporting Thunderbolt protocol utilizing the USB-C style port. Theoretically this should work if the external drive supports the Thunderbolt protocol.

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

Apple Thunderbolt Cable (2.0 m) - White - Apple

My guess is the external drive only supports the USB protocol.