Well, to topic, it has always bugged me that you need 3 Elder Scrolls and even though I have completed several times the Dawnguard main quest with both sides I can only remember using 2 of them, the one Serana has (the Sun one?) that you give to Dexion and then he tells you that the prophecy continues in more Scrolls but you only use 1 more (the Blood one from Valerica that you yourself read) that reveals you the location of the bow and after that you don't use them again.
So, in Dawnguard, why do you need the Dragon Scroll (the one you get in the ruins)?
Greetings.
PD: I may have messed with the Scrolls names but it boils down to telling that you need 3 Scrolls but you ever use 2.
PD2: Sorry for any mistakes (English is not my mother tongue (Spanish is)).