How to wire a 3-way switch and a RF Relay to control Lights
The energized relay going for weeks/months shouldn't be an issue.  The bigger question is why bother with a physical 3-way switch?  Just get two RF transmitters on the same frequency and put one in the shed in place of the switch.

Or get something like the Lutron Caseta Wireless Switch kit.  It comes with a switch and a remote relay.  It has a dimmer as an option if you want to dim the light as well.  And also has an optional hub so you can turn your shed light on and off from the Internet anywhere in the world.