How to determine what kind of front derailleur can be used on my recumbent trike?
Recumbent / trikes tend to use mountain-bike shifting equipment because they need the extra gear range. While there is a difference in cable pull between road and mountain sets,  front shifters on the whole tend to be much less picky than rear shifters about chain width because essentially  the limit for front shifting has stayed at 3 chainrings (aside from @Criggie with his 4 chainring setup). While it is optimal to try to match the chain width (speed), it is not a necessity. As always: Sheldon Brown

That means you can use pretty much any (Shimano) mountain bike triple front derailleur that fits. The main consideration for you will be how to mount the front derailleur. If you're using a clamp mount, you have to have one that can fit the tube where the derailleur mounts (the 34.9mm in the examples below are the seat tube thickness but many times the derailleur will come with a variety of clamps and shims to match different tube sizes). If you have braze-ons, they need to fit the braze-ons.

The other consideration is where the cable pull comes from (from the top or bottom of the derailleur as it would be normally mounted on a standard bicycle). Many front derailleurs allow for pull from either direction (dual pull) but some like the SRAM below are "top-pull only" or "bottom-pull only."