Electronics 2000

Hi,
I'm a new hobbist to electronics. I have made a bench PSU out of an old computer ATX. The problem I'm having is wiring an ammeter to the three out-power post's im using. I've wired the three outputs (3.5v, 5v, and 12v,) through a BBM rotory switch but how can I wire an ammeter in series to all three outs. Every time I try the ammeter feeds the other two posts with the supply voltage.
I'm stuck and think I'm missing somthing simple here.
THanks Chris.

I'm not sure what you mean by "out-power post's im using". Is that meant to be "power output posts that I'm using"?

If you are selecting any one of three voltages by means of a rotary switch then you'll have three inputs to the switch and one output. Wire the ammeter in series with the output from the switch.