As already mentioned, the gravitational forces arising from the interaction between the falling objects would be very small when compared to the weight forces acting on them due to their interactions with Earth. If you wish to include that in your calculations, then, since they are dropped from points 5m apart, you should also consider the fact that the value of g may be very slightly different for each one, and that the direction "vertically downwards" may also be very slightly different for each one (which would be true even if the Earth was a homogeneous perfect sphere). That's the problem with not neglecting negligible effects - you open up a can of worms!
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