How to find the volume of free space inside a pyramid of 4 spheres.?

There are 4 spheres, 3 making up the base with the other stacked on top in the middle. Each sphere has a radius of 1.85u. In the middle of all the spheres, there is an empty space. How can I know where this space ends (as it meets with empty space outside the pyramid), and how can I calculate its' volume?

Following David F's advise set up the

Volume of tetrahedron formed by joining centers =
sqrt(2)/12*(2*1.85)^3u^3=
5.969513298u^3
Volume of portion of tetrahedron which is transecting four touching spheres
4*鈭?dihedral angles of planes about the vertex)
-鈭慳ngles of plane triangle(spherical excess)
divided by total area of sphere times volume
4*(3*acos(1/3)-蟺)*3.4225u^2/4蟺*3.4225*
u^2*(volume sphere)
4*(3*acos(1/3)-蟺)/(4*蟺))*4/3蟺(1.85u)^3
=4.654044903u^3

Vhole=1.315468395u^3

Do you know how to calculate the calculate the volume of a segment of the sphere using the 1.85u value? ... (if not then i dont think you can solve this at your level)
... If so: use this to calculate one point of the pyramid, multiply by four as you have four points on the pyramid (as the are the same size the top should have same internal angle)
calculate the volume of an equilateral pyramid using a base length of 2r (r - 1.85u)
Now subtract 4*(one points internal circular volume)) FROM the pyramids total volume leaving the internal space as the resultant.

hope this helped.