V=4πr3/3, where r=radius, and r=D/2 where D is the diameter.
dV/dr=4πr2, dV/dt=(dV/dr)(dr/dt)=-3ft3/min,
4πr2dr/dt=-3, and when r=2ft, this is 16πdr/dt=-3, making dr/dt=-3/(16π).
dr/dt=½dD/dt, so dD/dt=2dr/dt=-3/(8π)ft/min. So the diameter is decreasing at the rate of 3/(8π)=0.1194ft/min approx.