$ABC$ is a triangle in the plane. Find the locus of point $P$ for which $PA,PB,PC$ form a triangle whose area is equal to one third of the area of triangle $ABC$.
1. **Assume the triangle is equilateral**: Let $ABC$ be an equilateral triangle with side length $l$ and area $S$. Let $P$ be a point such that the area of the triangle formed by $PA, PB, PC$ is $S'$. We are given that $S' = \frac{S}{3}$.

2. **Known relationship**: By an argument involving a $60^\circ$ rotation around $A$, it is known that:
   \[
   2S = 3S' + \frac{\sqrt{3}}{4}(AP^2 + BP^2 + CP^2)
   \]
   This relationship can be derived from geometric properties and symmetry of the equilateral triangle.

3. **Substitute $S' = \frac{S}{3}$**: We substitute $S' = \frac{S}{3}$ into the equation:
   \[
   2S = 3\left(\frac{S}{3}\right) + \frac{\sqrt{3}}{4}(AP^2 + BP^2 + CP^2)
   \]
   Simplifying, we get:
   \[
   2S = S + \frac{\sqrt{3}}{4}(AP^2 + BP^2 + CP^2)
   \]
   \[
   S = \frac{\sqrt{3}}{4}(AP^2 + BP^2 + CP^2)
   \]

4. **Express $S$ in terms of $l$**: The area $S$ of the equilateral triangle $ABC$ is given by:
   \[
   S = \frac{\sqrt{3}}{4} l^2
   \]
   Therefore, we have:
   \[
   \frac{\sqrt{3}}{4} l^2 = \frac{\sqrt{3}}{4} (AP^2 + BP^2 + CP^2)
   \]
   Simplifying, we get:
   \[
   l^2 = AP^2 + BP^2 + CP^2
   \]

5. **Use the circumcenter $O$**: Let $O$ be the circumcenter of the equilateral triangle $ABC$. We know that:
   \[
   \sum AP^2 = \sum (\vec{P} - \vec{A}) \cdot (\vec{P} - \vec{A}) = 3OP^2 - 2\vec{OP} \cdot \sum \vec{A} + \sum \vec{A} \cdot \vec{A}
   \]
   Since $\sum \vec{A} = 0$ in the case of an equilateral triangle centered at the origin, this simplifies to:
   \[
   \sum AP^2 = 3OP^2 + 3R^2
   \]
   where $R$ is the circumradius of the equilateral triangle.

6. **Circumradius of an equilateral triangle**: The circumradius $R$ of an equilateral triangle with side length $l$ is given by:
   \[
   R = \frac{l}{\sqrt{3}}
   \]
   Therefore:
   \[
   3R^2 = 3\left(\frac{l}{\sqrt{3}}\right)^2 = l^2
   \]

7. **Equality condition**: For the equality $l^2 = AP^2 + BP^2 + CP^2$ to hold, we must have:
   \[
   3OP^2 + l^2 = l^2
   \]
   This implies:
   \[
   3OP^2 = 0 \implies OP^2 = 0 \implies P \equiv O
   \]

Conclusion:
The locus of point $P$ for which $PA, PB, PC$ form a triangle whose area is equal to one third of the area of triangle $ABC$ is the circumcenter $O$ of the triangle $ABC$.

The final answer is $\boxed{O}$.