Imagine that of the 3 wires that come into the receptacle 2 are hot and one is common. They come from the same source. One of the hot conductors is hot from the source, the other hot conductor, however, is run through a simple wall switch.
In my house, I ran 12/3 through every receptacle box. So…2 hot wires “arrived” in each box----one that was hot all the time, and one that was on a switch circuit.
If I wanted that particular receptacle to be switched I used the switched conductor and the all-the-time hot conductor was simply wire nutted so that at the next box they all arrived the same as the previous box; 1 hot all-the-time, 1 switched, and one common. In other words, in each box 1 hot leg is used, (the one you choose) and one is just “passing through” via a wire nut.
I hope that makes sense.
We don’t want to confuse you, but there is one other way to do it, and it is actually a better way. On any given receptacle, you would likely only need/want one side of the receptacle switched—usually for a lamp.
On all receptacles there is a small tab that is essentially a jumper that joins the 2 sides of the receptacle together. Take a long nose pliers and remove the jumper. Now…there are 2 hot legs coming into the receptacle box and you can put one hot leg on one side of the receptacle and the other hot leg (the switched one) on the other side of the receptacle. Voila! You now have one side of the receptacle switched for the lamp, and one hot all the time for the clock.
It’s possible that it’s all in the same daisy chain—particularly if it’s in the same room. They may have run 12/3 to just the first or second outlets----the ones they intended to be switchable outlets----and then continued on with 12/2.
That’s short sighted IMO, because wire is cheap relative to labor, and you really want the flexibility of making other outlets switchable as the furniture layout changes.
We can’t see the room, and you haven’t given us a lot of info, but it seems that the outlets that are 12/3 are stitched—the ones that are 12/2 are not. If you went upstream and used the switched hot leg as the source for the 3 outlets that are using 12/2 they would all become switched outlets—all 3, and worse yet, both sides of the receptacle. That’s no good.
IME, you need 2 hot legs coming into each box; I hot, and 1 hot/switchable.
Without more info, I would guess you need to find that switched leg and pull a conductor into each box from that switched source.