Basically, diminishing returns occur when you use the same basic type of crowd control (CC) over and over again against the same target. For example, I can sap a target for 6 seconds. If I sap it again or a shaman uses hex when that sap expires, it will only last for 3 seconds. Sap and hex share the same CC type and shouldn’t be used right after one another.
Manaflask has a good list of crowd controls that share diminishing returns. It’s organized by the basic types:
If you look at that list carefully it will give you an idea of why some 3s comps work better than others. Having CCs that share the same diminishing returns (DR) limits your ability to chain CC. For example, RMD (rogue, mage, druid) can kidney shot, into a cyclone and into a polymorph. That’s 18 seconds of crowd control if that focus target doesn’t trinket or have another out.
I used to use the Last Second Diminishing Returns addon to track when I can use my CC on a focus target. I now use TellMeWhen for that. But you don’t really need either. Once you use your CC on a focus target expect that your teammates will continue the chain. Once that is over you will likely not have to worry about DR for our next CC.
Another way to do that is to keep rotating targets. Mage/druid comps often just rotate their CCs onto two targets.