Well, if we're to go with more than 2 games as the threshold for entry, that'd rule KotOR out, but would technically leave Gothic in. But since I only played the first two, I couldn't consider them for myself.
And I do rather agree that three should be the minimum, two not being so much of a "series" but maybe more a case of developers either really finishing the first game, getting in all the content they originally had in mind, or making the game they really wanted to make the first time around but couldn't, or on the contrary, just throwing something together to cash in on the success of the first, or reusing the assets to try to recover some of the losses. But three tends to be a different story, often with an actual intent of a series.
So, series that I played at least 3 different games from (not counting expansions and such)? May go with Quest for Glory there, for 1-3, since 4 quickly put me off (and I never tried 5, read reviews back in the day and said no way). But there's a lot of nostalgia there, doubt I'd still say so today. But it's that or none, wouldn't go with any other.
Yeah, Might and Magic 6 seemed great and also played 1 and 7, but abandoned both early on, so wouldn't put the series that highly based on that.
Then, let's see, Age of Wonders 1, 2, Shadow Magic, sure, nice enough, but not something to file under favorite really. Did play 3 Championship Manager games plus Football Manager 2005 but only clearly identified one of the CMs and anyway, nope. It's possible that I played 3 Civilization games, if you also count Call to Power then quite certainly, but didn't even identify them exactly again, other than 4, and nah. Divine Divinity, Beyond Divinity and Divinity 2, but only finished DD and it was nice and so was D2 as far as I got but BD seemed crap (though I stopped early, may return someday, who knows) and actually DD may worm itself into favorites, albeit not towards the top, but not enough. Heroes of Might and Magic 1-4 and Tribes of the East, and unlike most I did really like 4 but as a result the rest were generally frustrating. Larry 1-3, 7 and 8, but no way. Monkey Island 2-4, but only finished 4 and definitely not. And very likely that I played at least 3 Worms games but only clearly identified one and nah.
