Let me explain what I was trying (still in the works) to accomplish with this setup. I didn't like the fact that all the other barrel setups it went straight into one barrel. Thus you are always dependent on having that one barrel there and everyone states they need cleaning eventually that really isn't cool to have your most important factor out of the picture. The other reason I've done it this way is to filter out the water partly before it enters the storage (aka barrels) I haven't come up with a way to do this yet so if another creative person comes up with something I'll add it here!
I used a plastic Tidy cat litter container so any plastic one will do for this. Use the male and female 2" adapter for this part. Cut a hole in the center of the container to screw in the male adapter from the inside. Make sure to put pipe tape on the threads close to the middle of the fitting and some marine caulk. On the female adapter put marine caulk on the inner threads and screw the male and female adapter together. Put a bead of marine caulk on the inside of the container and on the underside to create the seal. On the inside of the container put the 3" drain on top of the male fitting. (Note: i had to cut the inner part of the drain some to get it to fit over the 2" smooth male fitting. In the cover of the litter box cut 4 holes to let the water flow in. Cut a piece of window screen to wrap around the cover so when you put the cover back on it seals and secures the window screen on. I also put a bead of caulk around the top edge to secure it more.
This setup as you can see from the 2nd picture will then connect to the 3 way or T 2" fitting to go to the barrels.
Now for the part that I'm still not sure what way to go. I want to be able to filter the water before it enters the barrels. I want to put a pool filter in but I'm not sure if it will drain through the filter with out pressure. I'm trying to do this with out power at the moment to let gravity do its work. Any ideas will be accepted for this part.
Either way I'd recommend an initial catchment system to get most of the junk out before it goes in the barrels. If you are doing a simple 1 barrel system you could skip this step but I'd still recommend to do this so you have less cleaning to do later.