That sux I have Hulu also, but hate how they don't keep the whole season up at a time. You basically have to keep up within 5-6 episodes....Missing out on AoS season 2 because of this right now also....
I urge you to start Flash season 1 when you start Arrow season 3, and flip-flop between the shows/episodes (Flash 1, Arrow 1, Flash 2, Arrow 2, etc). If you watch Flash season 1 now, it may spoil certain plot points in Arrow.