After a lot of tinkering, I finally figured out the solution myself.

There are two options:

Option 1: Save the file as a TXT instead of CSV. Then, open Excel (just Excel, not the file itself) ... and in Excel go to File -> Open and open the TXT file. There, you can specify a desired delimiter, and it will split the content into three columns.

Option 2: This is even simpler:
db2 "export to users.csv of del modified by coldel0x3b select userid, name, description from users"
This way, everything is done automatically. It directly writes the data into columns "A," "B," and "C."