read -r exp awk "BEGIN { printf \"%.3f\n\", $exp }"

• BEGIN keyword tells AWK to execute the code without any inputs files or stdin
• Does a shell expansion of $exp and evaluates the expression to 3 d.p.
• On a side note the backslash after printf is to escape the double quotes so that awk can treat that as a literal double quote.

read -r expression

evaluation={expression}" | bc)

printf '%.*f\n' 3 "$evaluation"

read exp

result=exp" | bc -l)

printf "%.3f\n" "$result"

Just another way using here strings "<<<"

read expression result=expression") printf "%.3f\n" "$result"