I have a square of length $2n^2$, could it be possible to fill it by small small squares triplets? well, I am not able to guess how to proceed. please help
With length $2n^2$, the area is $4n^4$. Your triplet has three boxes, so a necessary condition is that $n$ is divisible by $3$.
To show that $n$ divisible by 3 is sufficient: join two triplets together to form a $2\times 3$ rectangle. If $n$ is divisible by $3$, we can line $2n^2/3$ rectangles end to end to form a row that is $2\times 2n^2$ in size. Then stacking $n^2$ of them you get the square.