When you are trading in One-Way Mode, your trading pair is open in one direction long or short.
Hedge Mode allows you to keep a long and short position at the same time for your trading pair.
Hedge Mode in 3Commas
In the My Exchanges tab, any accounts that support Hedge Mode and One-Way Mode will show a new button reflecting what mode you are in. You can click on it to toggle the mode.
When you click on your account, you will have another toggle button to change between modes.
Note: Before changing modes you have to close any open orders or positions. Otherwise the system will generate an error message.