There is no need to find the circumference.

Make a simple sketch.

Code:

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O 0.6

You'll find that the angle the string does with the horizontal to be:

$\displaystyle \theta = cos^{-1}(\dfrac{0.6}{3})$

Then, resolving vertically, the vertical component of the tension is given by:

$\displaystyle Tsin\theta$

Since the bob is in equilibrium vertically, the resultant force = 0.

The downwards force acting on the bob is given by:

$\displaystyle F = mg = 3 \times 9.8 = 29.4\ N$

Hence, $\displaystyle Tsin\theta = 29.4$

Solve for T.