How to get multiple results from SWITCH() function?

The logic behind switch is this:

if value equals X than "show me this", otherwise

if value equals Y than "show me that", ...

What you've wrote is this:

if value equals X than "show me this", otherwise

if value equals X than "show me that", ...

However this cannot work since X cannot equal 2 conditions at the same time. You cannot have "CRT" 3 times and "% Share Volume" also 3 times.

You need to create 3 separate switch functions:

Upper Bound = 
switch(
    SELECTEDVALUE(Range[Metric]),
        "CRT", 'Verizon Call Reason'[YMax CRT],
        "% Share Volume",'Verizon Call Reason'[YMax V%],
        0
)

Lower Bound = 
switch(
    SELECTEDVALUE(Range[Metric]),
        "CRT", 'Verizon Call Reason'[YMin CRT],
        "% Share Volume", 'Verizon Call Reason'[YMin V%],
        0
)

Average = 
switch(
    SELECTEDVALUE(Range[Metric]),
        "CRT", 'Verizon Call Reason'[Avg CRT],
        "% Share Volume", 'Verizon Call Reason'[Avg V%],
        0
)