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 "Trading" and "Margin" only are selected.
Any funds you wish to borrow and repay for Margin trades needs to happen on the Binance.com website; when 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 (it only had the API released recently by Binance) and therefore the Product Development Team have not had time to plan development tasks to add it as a 3Commas feature. If this changes, we will announce it to all users by email and the 3Commas Telegram Notification Channel.