"The first three have books."
Translation:Die ersten drei haben Bücher.
It's an adjective, and is governed by the rules for adjective endings. jess1camar1e gave me this lovely memory aid in another comment section:
"-Big 3 get an -e (der, die, das) der alte Mann, das kleine Kind, die schöne Frau
-Changin' gets -en (plural and case changes) den alten Mann (accusative), der schönen Frau (dative), die kleinen Kinder (plural)
-No 'the'? Adjective takes over (no 'der' word or just an 'ein') Kaltes Wetter gefällt mir nicht (das Wetter). Ein guter Mann ist schwer zu finden (der Mann).
Now the only tricky part is knowing which 'the' word your noun has :)"
(changin' means the "der" word differs -- has changed -- from its nominative singular form.)
See the entire discussion and a few more examples here: http://www.duolingo.com/comment/556140
I think it is because we are talking about three people or three objects, so we use 'ersten' because it's plural instead of 'erste' (singular), in Enlgish you don't have to put the adejctives into plural or singular, it is the same, but in Spanish and German, you do have, maybe that's a possible reason, or maybe I'm wrong.
I got this wrong. I think it is because erste is describing the subject of the sentence. "the first three" which is plural and adjective endings for the plural case are en (http://german.about.com/library/weekly/aa030298.htm)