Oh btw, here's an easy way to get monitoring on 12v3 and 12v4 too.
Buy a connector block of the type used in mains application for connecting together wires that have been spliced. Buy four relays. You will also need a couple resistors and a switch, but you do have those around don't you.
Stick the probes in the connector block and secure them. On the other side wire the outputs of the PSU thru the relays. Four wires come in, only two come out.

The relays have to be two of the NO (normally open) type and two of the NC (normally closed) type, so that when you toggle the switch they disconnect two of the rails from the scope and connect the two others. Power to the circuit can be taken from the PSU under test itself, it'll use next to no power so it won't influence your results. Want to flip thru more rails? Add more pairs of relays. You get the idea.