Filter Valve Settings

This may be a silly question, but I just bought a house with a pool and I'm slowly learning what to do as I go.

My question is about the valve options by the pump/filter. There's an option for all 3 ports to be open, the skimmer only open (which I use for vacuuming), and the option to close the other two ports in the pool.

My question is, when I add chemicals to the skimmer, do I set it on the "skimmer only" option to make sure it has enough suction to pull it all through? Or should I leave all 3 ports open?

I'm not sure if I'm using the right terms for this as I haven't been able to find anything online about it.

Any help would be greatly appreciated. Thanks!

Not a silly question, at all.

But you'll have to include exact multiport valve make / model (if that's what you mean) or pictures. You can email pictures to me at poolforum@gmail.com, or post them yourself using Webshots, Picasa, Flickr, Google Drive, or whatever.