Well, it's pretty much a matter of taste. I guess it also depends on what other RPGs you liked and disliked and what you expect from them in general.

I didn't expect Shadowrun Returns to be the next big RPG, I was just glad that it was something different from the usual fare. I actually enjoyed it more than the first Baldur's Gate or Icewind Dale, depite being "dumbed down", made for mobile devices, linear, short, somewhat predictable or what else it is criticized for, because I found the setting and atmosphere more engaging, and I also enjoyed the turn-based combat with modern weapons and cover, perhaps because I've hardly played anything comparable to it, since I'm usually not interested in the setting or other mechanics of similar games that offer it (XCOM maybe?). I also value the experience of a story in a cool setting with good atmosphere just as much as - or sometimes even more than - complex and challenging gameplay, so I didn't mind that it was very accessible even for beginners. It's not among the very best RPGs I've played, but definitely among the ones I quite enjoyed.

The experience of Avogadro6 and other players seems to differ from mine though, resulting in the opinion that Shadowrun Returns is an inferior RPG that you can or should skip in favor of Dragonfall. I agree that Dragonfall is much better than Returns, but personally I'm glad I didn't skip it, since I had fun with it nevertheless, and there aren't that many games like it to choose from.

What I can say is that if you like Returns, you will also like the other two, while it's not guaranteed the other way around. So if you're ready to gamble 2.69 EUR, you could start by getting Returns, play a bit of it during the next 2-3 days and then decide about the other ones. If you hate it, the series is probably not for you, but you only lost 2.69 EUR. If you see potential in it but you're somewhat disappointed by how little of it is achieved, try Dragonfall next. Or if you already like what you see, just get both of the other games.