HL from ep3 to ep1 and 2

[SophiaSeed]

a friend of mine currently owns and plays ep3 online right now. She is planning on getting a used or retail version of ep1 and 2, and is wondering if an ep3 serial will work on an ep1 and 2 game. I think it's a no, but please tell me just the same.

[Arislan]

Yes, and they are interoperable, to boot. Using one HL, you can play both Ep3 and Ep 1 and 2 using the one HL. Just enter the currently used HL into the other game, and you are good to go.