The game itself is very Trial and Error, you will be using Quick Save and Quick Load a lot, which isn't as bad as it sounds, to complete missions and have everything run smoothly. You do not need to play any of the other older Desperados games in order to understand the story, this game is rather self-contained and doesn't require any prior knowledge of the other games thus making it easier to get into.
The first thing I'd like to highlight, is despite Desperados 3 being the 'Third' main entry into the series, the game is actually a prequel/soft reboot.
Desperados 3 is a Real Time Tactics game developed by the same group that did Shadow Tactics: Blades of the Shogun, so if you've played that you'll feel right at home with Desperados 3.