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I suggest that you had better not separate planes.
Because it is quite easy to create bad signal return paths.
This creates a slot antenna that radiates and receives noise.
It also contributes to ground bounce.
You need to make sure that you actually have a component on the schematic that connects them. They won't show as separate nets when you export the netlist if you have them connected by a schematic wire. So this is really important to keep in mind.
You can create three separate, non overlapping filled zones on your ground layer. And let the three zones set to net AGND, DGND, and GND seperately.
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