The likelihood function is .

Now solve for in terms of and .

I get but I'm known to make careless mistakes.

Now substitute and .

If you quote any reply you'll see the latex code. For what it's worth, here's the code I used to generate :

[tex]f(x_1, \, x_2, \, x_3) = \frac{x_1^3 \, x_2^3 \, x_3^3}{6^3 a^{12}} \, e^{-\frac{(x_1 + x_2 + x_3)}{a}}[/tex].

No doubt a latex guru will chip in on how to make it look better.