Assuming no reflection or absorption of sound, the intensity of sound from a point source follows an inverse-square law: if the source is at point $s$, the intensity $I(x)$ at point $x$ is $C/\|s-x\|^2$ for some constant $C$, where $\|\cdot\|$ is Euclidean distance. Suppose $s$ and the three microphones $m_1$, $m_2$, $m_3$ are all in the plane, with $m_i$ known. Then the ratios of intensities give you two equations $I(m_1) \|s - m_1\|^2 = I(m_2) \|s - m_2\|^2$ and $I(m_1) \|s - m_1\|^2 = I(m_3) \|s - m_3\|^2$. Each of these defines a circle in the plane (or a straight line if the intensities happen to be equal). Usually, if they intersect the circles will intersect at two points, so there are two possible locations for the source.
EDIT: In your example let $m_1 = (0,0)$, $m_2 = (2500,0)$, $m_3 = (2500,1)$ with (assuming the numbers you give refer to intensities) $I(m_1)=75, I(m_2)=20, I(m_3) = 10$. Then if $s = (x,y)$, $75 (x^2 + y^2) = 20 ((x - 2500)^2 + y^2)$ and $75 (x^2 + y^2) = 10 (x^2 + (y - 2500)^2)$ become
$ \eqalign{(x + 10000/11)^2 + y^2 &= 375000000/121\cr x^2 + (y + 5000/13)^2 &= 187500000/169\cr}$ which correspond to a circle of radius $\sqrt{375000000/121} \approx 1760.446975$ centred at $(-10000/11, 0)$ and a circle of radius $\sqrt{187500000/169} \approx 1053.312611$ centred at $(0, - 5000/13)$. These intersect at approximately $(831.8160827, 261.5652866)$ and $(115.4863013, -1431.577834)$.