3 LEDs / 1 POT - fade each LED in turn as POT is changed in value ?
A differential pair something like the following could cross-fade two LEDs:







simulate this circuit &ndash; Schematic created using CircuitLab

This is because the emitters are connected so if you turn Q1 on, Q2 will turn off (because the Q2 Vbe will drop because the voltage at the emitter of Q2 will go up).

So you position the pot at ground and then adjust the R3 R4 bias until the D1 LED gives the desired brightness. Then as you turn up the pot they should cross fade I think. But this circuit is just a back of a napkin scribble. I would simulate it first. Then you might take what you learn to figure out how to control 3 LEDs somehow. Then build.