Decide on a rectangular (Cartesian) co-ordinate system to use. For example, for the 3 legged stool the origin could be at the CM of the seat with one axis pointing downwards parallel to the legs.
Divide the object into component parts which are regular 3D or 2D shapes for which it is easy to locate the CM. For example, cylinders, cuboids, spheres, cones, pyramids, circles, rectangles, triangles. Find the mass $m_i$ and co-ordinates of the CM $(x_i, y_i, z_i)$ for each component part in your co-ordinate system.
The CM of the composite object $(\bar x, \bar y, \bar z)$ is the sum of the co-ordinates for each component shape, weighted by the fraction $\mu$ of the total mass in each component part :
$$\bar x= \mu_1 x_1+ \mu_2 x_2 + \mu_3 x_3 +...$$
and similar for $\bar y$ and $\bar z$, where $\mu_i=m_i/(m_1+m_2+m_3+...)$.
In step 2, if part of a regular shape is missing, treat the shape as two separate parts : (i) a complete shape with positive density and (ii) a missing shape with the same negative density. The missing shape will have a negative value for the weighting factor $\mu_i$.
See Calculating the Center of Mass.