Your question is using the term "return" in an unusual way. Usually return refers to rate of return which in this case is the interest rate. For this problem, I will interpret return as the dollars earned as interest.

Also the formula you have for continuous compounding is not correct. A=P*e^(i*t) where A = Account value, P = initial balance, i = interest per year, and t = time in years. The symbol ^ means raised to the power of.

Let A(t) be the account value at time t in years.

Let r(t) = A(t)-3000

Annual compounding

A(6) = $3000*1.07^6=$4502.19

r(t) = $1502.19

Monthly compounding

A(6) = $3000*(1+.07/12)^(6*12) = $4560.32

r(t) = $1560.32

Continuous compounding

A(6) = $3000*e^(.07*6) =$4565.88

r(t) = $1565.88