WH2 improved upon the gameplay of WH1, with further improvements coming over the almost 4 years since the second game released. It also integrated almost all of WH1's content. As a result, there are few reasons to play WH1 now; WH2 is simply better. The same will likely ring true with WH3. But it still makes sense to buy the first two games as they will unlock a lot of content in WH3. If you only have WH3 then you'll only get to pick from Kislev, Cathay, Korne, Nurgle, Slaanesh and Tzeench. Owning WH1 will unlock Empire, Dwarfs, Greenskins and Vampire Counts, while WH2 will unlock High Elves, Dark Elves, Lizardmen and Skaven.