In Bash and other shells that support it, you can do
mkdir ~/Labs/lab4a/folder{1..3}
or
mkdir ~/Labs/lab4a/folder{1,2,3}
Other options:
mkdir $(seq -f "$HOME/Labs/lab4a/folder%03g" 3)
mkdir $(printf "$HOME/Labs/lab4a/folder%03g " {0..3})
Which will give you leading zeros which make sorting easier.
This will do the same thing in Bash 4:
mkdir ~/Labs/lab4a/folder{001..3}
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