I loved the Three Sisters Falls trail! However, I will say that is is more challenging than many other San Diego hikes.
There is a very steep section with deep sand that will definitely get into your Nikes. You will get dusty and should wear sunglasses to prevent sand from getting in your eyes. It is also quite confusing getting to the falls because there is no clear signage (although some people have spray painted arrows on rocks). Best to go when there are others around so you can use the group-buddy system.
Bring a camera, lots of water, and a swim suit! (And watch out for snakes!) If you have time, go to Julian and get some pie a la mode.
Check out some of the photos I took with my iPhone 6s: