Start with Risen. It's very similar to Gothic and G2, just much easier to get into and generally slightly weaker. If you enjoy this type of game enough to commit to the real thing, download JemyM's FAQ and start with Gothic 1. (You can map the controls to an old gamepad, if it's more comfortable to you.) The learning curve is 3*5 minutes if you know where to look - or hours if you try to find everything out on your own.
Then play Gothic 2 vanilla all the way through, but don't try all the different solutions in chapters 1 and 2. The interface is an optimized version of Gothic's so you simply go on without reading the manual.
Then play G2 Gold until the end of chapter 2. That's where the fresh NotR material ends, so it's a convenient exit point. The overlap is something between 5 hours and 20 hours, depending on your playing style. Look under every rock and don't waste any skill points. NotR is very hard.
Gothic 3 is a different game, more similar to the Elder Scrolls. Play it whenever you feel like it, but make sure to patch it to the latest CP (the upcoming 1.75) and read the pdf for the CP before you start. It's a long game with one of the best RPG worlds ever, but a couple of serious issues. Hit or miss.
Ignore the G3 add-on.