$3.0$ mL. The precision of the measurement of the first sample is the limit.
Work Step by Step
The first cylinder contains 3.0 mL of water (This is actually between 2.9 and 3.1 because of the precision of the measurement. ) The second cylinder contains 0.30 mL of water (between 0.29 and 0.31). The sum of those two is $3.0+0.30=3.30$ mL of water. However, in addition, the result has the same number of decimal places as the least precise measurement used. So, the result should be written as $ 3.3 $ mL as $3.0$ has two significant digits.