How to express Switch()

This is actually a case where a nested IF() is more suited than SWITCH(). SWITCH statements compare definite answers so Stock < 0 would be evaluated to either true or false, so unless the output of {StockDO} is a true/false field (like a checkbox), this setup will not work for you.

Your best bet is the formula you already have, probably, with (presumed) typos fixed.

IF({Stock}<0,":x:",IF({Stock}=0,":heavy_minus_sign:",{Stock}))

Hopefully you mean columns. Comparing rows with formulas is not possible unless the rows are linked together.

IF({Stock}<0,":x:",IF({Stock}=0,":heavy_minus_sign:",{Stock}))

the goal can be achieved using binary logic, like
SWITCH(
({Stock}=0)-({Stock}<0)
,1,‘ :heavy_minus_sign: ’
,-1,‘ :x: ’
,0,{Stock}
)

but that’s more complex and not needed in that case,
while nested IF is simple and obvious.

thanks for the answer i just wanna know is there alternative, instead making a lot unique column