I agree that you should rent a car. As both Gabriel and Yossie said, you CAN do them all in one day - it really depends on how durable your palate is, and how much you can taste before you've had enough.
At Flam, for an extra fee, they will put on a "show" with a table set with cheeses and bread. When we visited in March, we sat with Yisrael Flam for close to two hours of wonderful tasting and conversation. I've had similarly good experiences at Ella Valley, but it's been a bit variable.
Although I like Tzora's wines, every time I've visited, they seemed rushed, and the visits have never compared to the other two.
(If you're planning on being in that area, Castel is another option, as they are just up the road from Flam. The visits there are always great, but you have to commit to purchase a case if you want to taste there.)