Genetic algorithm to find the minimum of a three-variable function
Better organisation

You program in its current state is one huge chunk of snippet with all logic inside it. You should consider splitting it into separate smaller functions. The limits on \$ x, y, z \$ are preset. Consider putting them as a GLOBAL_CONSTANT.
Ideal import order (according to PEP8) is


standard library imports
related third party imports
local application/library specific imports


os/collections etc. are standard library, and should be imported first, followed by numpy.
pythonify your code

x = args[0]
y = args[1]
z = args[2]


can be expressed as:

x, y, z = args


similarly

manager.list([np.inf for i in range(child_num)])


can become

manager.list([np.inf] * child_num)



I am not sure about how numpy.argmin works, but I think using None instead of np.inf could be better in the sense that you might extend your program to also find local/global maxima along with minima points.