3Commas currently offers Binance Margin support for Cross margin only at present.
This can be used for Smart Trades and Bots by linking a Binance (Margin) account via the My Exchanges page in 3Commas - you'll need to create a new API Key & Secret on the Binance.com website and ensure "Enable Spot & Margin Trading" only is selected (if you want to connect only Spot and Margin accounts):
Any funds you wish to borrow and repay for Margin trades need to be borrowed on the Binance.com website itself; after you borrow margin, this will then appear in 3Commas to be used on Smart Trades or Bots.
3Commas does not currently support Isolated Margin mode, but our Product Development Team has plans for development tasks to add it as a 3Commas feature. If this changes, we will announce it to all users by email and on the 3Commas Telegram Notification Channel.