There are a couple of ways to do this. One is to do as Tim said, and place separate sheet instances without using repeat. You should be able to place the instances on different sheets without trouble if you need to (like if there's a bunch of instance-specific surrounding circuitry. Then you can name the nets from outside the sheet, and make sure your net name resolution rules allow the higher-level sheet names to override the lower-level ones.
The other is to use repeat and to rely on the connectivity/net naming settings. This means that if you place the sheet with "Repeat(Foo,1,3)" then the net "12V_LOW" on the sub sheet will be translated into something like "12V_LOW_Foo1", "12V_LOW_Foo2", etc, on the different sheets, depending on your hierarchical naming rules. I don't know if this will be close enough to what you want. With repeat, you'll need to use the repeated sheet entry syntax to bring out a bus of signals to each instance, and then break those busses out to be able to get at the signals to each channel. It gets kinda messy, TBH.
Note that, either way, power ports default to global connect (IIRC), so you can't use them in your subsheet for signals that should be separate per-instance unless you change that. But you can bring that signal out of the subsheet via a port, and then connect to a power port in the higher level sheet.
Repeat really works best when every channel is identical in both form and function, like input channels on an audio mixer or something, if you are bolting on additional functionality that varies between one channel and the next it gets clunky and you're probably better off doing separate sheet instances.