Why did Hollyleaf die in Sunrise?

I have a question.........if Jayfeather,Hollyleaf and Lionblaze are supposly the three cats in the prophecy.............then what about Dovepaw the summary I read said she later found out she was the third cat........does that mean Hollyleaf was supposed to die and was never the third..........and she had to die when Dovepaw was born,the true third cat in the prophecy......and if so is she their sister too? UGH I NEVER READ THE WHOLE SERISE AND IM CONFUSED DX and im new on this.......:P