The solutions to your diophantine equation known due to Nagell. I have in front of me Paulo Ribenboim's "My Numbers, My Friends" from which I quote:  

> **Theorem.** If $m > 2$, the only non-zero solutions of $X^2 + X + 1 = 3 Y^m$ are $x = 1$ and $x = -2$. If $m = 2$, there are also the solutions
$$x = \pm \frac{\sqrt{3}}{4}\left((2+\sqrt{3})^{2n+1} - (2-\sqrt{3})^{2n+1}\right) - \frac{1}{2}$$
for $n =0, 1, \dots$. 

The reference from Paulo's book is: 

> T. Nagell. _Des équations indéterminées $x^2 + x + 1 = y^n$ et $x^2 + x + 1 = 3y^n$._ Norsk Mat. Forenings Skrifter, Ser. I, 1921, No. 2, 14 pages. (= 1921a at the end of Chapter 7.)