Hey renas.

If these points are in flat space (i.e. in the typical three dimensional geometry with x, y, and z axes) then you can use the cosine rule where the three sides of a triangle are related by A^2 = B^2 + C^2 - 2*B*C*cos(B,C) where cos(B,C) is the angle between sides B and C.

You can find cos(B,C) by forming vectors with the other two, take the dot product and divide by the product of the lengths of the two vectors.

Using all of this will give you the distance of BC.