Step 1:
If a constant value is added to every observation, the arithmetic mean also increases by the same constant.
Step 2:
Suppose original mean is:
\[
\bar{X}
\]
If each observation is increased by $K$, new observations become:
\[
X+K
\]
Step 3:
The new arithmetic mean becomes:
\[
\bar{X}+K
\]
Thus, the mean increases by $K$.
Step 4:
Therefore, the arithmetic mean:
\[
\boxed{\text{gets increased by }K}
\]
Hence, the correct answer is:
\[
\boxed{\text{(3) gets increased by K}}
\]