Hi Ahmad,
Transit Duo has 2x cellular modems and it can use 2x SIM cards simultaneously. In case you will insert 3 SIM cards, then one SIM card will be used as a backup for one of the cellular modems.
To answer your questions about SpeedFusion bonding need to know which SKU you gave purchased. I suppose the model had to come with a Prime care, so for the first your you get 1TB SpeedFusion Protect data to use for bonding (forum post).
To set up the Bonding you will need to connect to the device configuration WEB UI. At the top bar you will see a tab SFC Protect where you can enable SpeedFusion bonding.