Use the str.translate() method to apply the same translation table to all strings:
removetable = str.maketrans('', '', '@#%')
out_list = [s.translate(removetable) for s in my_list]
The str.maketrans() static method is a helpful tool to produce the translation map; the first two arguments are empty strings because you are not replacing characters, only removing. The third string holds all characters you want to remove.
Demo:
>>> my_list = ["on@3", "two#", "thre%e"]
>>> removetable = str.maketrans('', '', '@#%')
>>> [s.translate(removetable) for s in my_list]
['on3', 'two', 'three']