I have a physical copy of WBC3 and despite some balance issues i really like it, but after seeing the offers im wondering if buying the second and maybe the first one, however i wanted to know what the fans of the series say about.
Since I originally played the three games in the release order, I love all three. I'm not sure you would get much new from the first two, if you played the third already, since the series kept building upon what they already had, keeping a lot of the graphics, sounds and stuff from the first and second game.
The first two games may look on first sight as lesser versions of WBC3, but they have some differences on hero building, tech trees and other stuff.
On the first game you have ships, which were removed in both sequels, so that can make gameplay a little bit different on the maps where available.
The first game has a campaign (for a human hero only), it has some dated cinematics (maybe were even dated back then) and in the last part the story splits into a good and an evil path (you're allowed to progress both with the same hero and see both endings).
The second game's campaign is a world conquer with no story, near the end the surviving sides offer you gifts to stop your campaign, from inventory items to retinue troops and even marriages, or you can conquer everything and gain a powerful orb, then you can reset the campaign and start again if you want.
I think the first game is too simple and offers little other than the campaign. The second game is my favorite, but that could be nostalgia talking, as it was the one I played the most back then. A case could be made about the second game being worth trying even if you already have the third one, but I really don't know if I should recommend it, maybe you can see some review or gameplay on YouTube before deciding if it's worth buying.