RC Filter Parallel Potentiometer
It's not the best example of a three way splitter (I doubt you'll see something like this inpractice, it's just an example), but 680 is less than 10k, which means the cutoff will be little influenced. The same for the others where the cap is in parallel with the pot -- at cutoff, the impedance is less than the pot (~1.6k). That's not to say they will not be influenced, here's how the three outputs look like when there's no potentiometer (R=1G Ohm), compared to 10k:





Given @user7286370's comment: No, they combine, but as you can see in the picture, the effect of one large resistor in parallel with a small one will result in a new resistor that's slightly less than the smallest of the two. Consider the formula: \$\frac{R_1 R_2}{R_1+R_2}\$. If \$R_1\$ is much greater than \$R_2\$, what happens? Here's what happens when you put a \$10\Omega\$ resistor in parallel with another varying from 1 to 100:



If the resistor is less than 10 (see the graph), the combined resistance drops significantly. If they are equal, 10 each, the resultant is 5, and as the resistor goes to 100 you can see the resultant value going up, towards 10, asymptotically. That means that, the higher the parallel resistor, the less impact it has.



Here's the equivalent schematic for the highpass case:



As you can see, the pot effectively becomes two series resistors, with their values set by the wiper position (here, from 0 to 1). At the output you have the input impedance to the next stage (considered larger). From R1's perspective, it "sees" the two series resistances, thus the whole potentiometer, which, as determined above, does not influence too much R1's value. And, at the output, you have R2 with R3 forming a variable voltage divider.

Not sure what you meant to say with this: "does not act as a 'volume' control for a fixed value filter", but buffering is the recommended way to do it because, as you can see in the 1st picture, despite the large value of the pot, it still influences the filter, even if only by a bit.