User blog comment:TheSonofNeptune/Arrow/Flash teamup show in works?/@comment-30384903-20150227070619/@comment-24380534-20150321180556

I got two main theories to explain Caity Lotz reprising her role. The first one is to make it so during the 5 months between the season 3 finale and the Flash pilot Sara found Multiplex, the metahuman who could make clones and somehow convinced him to make a clone of herself. Thus that's what was killed by the arrows in the season 3 premier: a clone.

The second theory is longer and I wrote it on a blog. Here is the link: http://arrow.wikia.com/wiki/User_blog:San_Layn/Flashpoint_Adaptation_for_the_CW_New_Universe