What are values of fundamental frequency and the harmonics for the below signal g(t)?
In general, Fourier coefficients are given by the formulas:

$$a_n = \frac{2}{T}\int_{x_0}^{x_0+T}g(x)\cos\left(\frac{2\pi nx}{T}\right)dx\\
b_n = \frac{2}{T}\int_{x_0}^{x_0+T}g(x)\sin\left(\frac{2\pi nx}{T}\right)dx$$

For any integrable \$g(x)\$ on the interval \$[x_0, x_0+T]\$. For periodic functions this interval can correspond to the period of the function, such that the Fourier series will be perfectly equal to the function itself. But the case of your example is very simple - it's just a sum of three cosines, so the \$a_n\$ terms can be calculated directly and there would be three of them. \$b_n\$ would equal to zero, as there is no sine terms.