Given the heights of tree seedlings: 143, 139, 161, 144, 160, 158, 160, 144, 154, 138, 144, 159. Find the mean, mode, and median height.

Sum of heights $= 143 + 139 + 161 + 144 + 160 + 158 + 160 + 144 + 154 + 138 + 144 + 159 = 1804$.
Number of seedlings $= 12$.

The mean height is $\boxed{\text{150.33 cm}}$.

The data set is: $143, 139, 161, 144, 160, 158, 160, 144, 154, 138, 144, 159$.
Count the occurrences of each height:
$138: 1$
$139: 1$
$143: 1$
$144: 3$
$154: 1$
$158: 1$
$159: 1$
$160: 2$
$161: 1$
The height that appears most frequently is 144.
The mode is $\boxed{\text{144 cm}}$.

$138, 139, 143, 144, 144, 144, 154, 158, 159, 160, 160, 161$.
There are 12 data points, which is an even number.

The median for an even number of data points is the average of the two middle values.
The middle values are the $6^{th}$ and $7^{th}$ values: 144 and 154.

The median height is $\boxed{\text{149 cm}}$.