Could you list the chronological order of which they can be played bearing in mind I primarily play on PC.
In terms of pure chronological release
1. Trails in the Sky 1, 2, 3 - all available on pc, (1 & 2 also available on PSP/VITA)
2. Crossbell duology- this is tricky, as its never been released out west. The only solution is to learn Japanese, or It is possible to download the Chinese PC release and some unofficial English fan translations of dubious quality, but supposedly there is a better fan made translation also in the works. Alas, we may never see these "officially" come over. I personally never played it as I don't do PC gaming (or the whole mod scene), but made myself aware of its key events through online resources and whatnot.
3. Trails of Cold Steel- 1 & 2 are out now for PC & Vita, and recently just released enhanced version son PS4. 3 & 4 are out only in Japan right now, but PS4 is getting part 3 out west in September, and hopefully they will announce 4 sometime soon as well.
Ideally, it would be best to take these in via the above order. In terms of in-game timeline, the Sky games happen first, then the 2 Crossbell games and the first two Cold Steel games happen concurrently in their respective region, with Cold Steel 3 & 4 happening after and containing characters from all games that came before (but still centered around the Cold Steel protagonist)
However, each series CAN be picked up by someone new coming in and enjoyed for the most part by starting with each 1st respective game since they do introduce new protagonists/stories/regions(though some minor references/side characters might not have the significance they could have because you might have seen them make an appearance before in a previous game). Just make sure you start with the first game of each respective series (don't even think about starting with Trails 2 or Cold Steel 2 since both are direct sequels that take places right where previous game left off)