Outbox is different. You only need three otherwise other clients have to pull from all the relays for the same note. 

You can set general relays to simply read from to get more content. The inbox and outbox are to simplify which relays clients need to push/pull from. 

In Amethyst for example I set a few relays in outbox and inbox then added like 7 more in general but only have them set to read.