Hint:
Let $\beta$ denote the probability of a type II error under the assumption that $\mu = 3$. This means $T < 2$ although values up to $\mu = 3$ can be assumed. Then
* $\beta = P_{\mu = 3}(T <2) = P_{\mu = 3}([0,2]\times[0,2])$ where
* $P_{\mu = 3}$ is the uniform distribution on the square $[0,3]\times[0,3]$.
Some more info:
Note that you are dealing with squares, as the sample consists of two (independent) random variables. So, you need to consider the squares $2^2$ and $3^2$. Then, you get the correct results.
Maybe you may draw the region $T<2$ on the square with side length $3$ to get a visual grip of what you are calculating.