Rate of Change of Volume?

A trough 12 feet long has a cross section in the form of an isosceles triangle (with base at the top) 4 feet deep and 6 feet across. If the water is filling the trough at the rate of 9 cubic feet per minute, how fast is the water level rising when the water is 3 feet deep?

http://pic90.picturetrail.com/VOL2363/11813548/209...

So, I found the sides of the triangle using the Pythagorean theorem.

And I know that V = 1/2*B*H*L

So wouldn't the rate of change equation be

dV/dt = 1/2*B*H*dL/dt + 1/2*B*L*dH/dt + 1/2*H*L*dB/dt

If so, then I know that I'm finding dV and I'm pretty sure that dH is 9, but what are B, dB/dt and dL/dt?

Or am I going in the wrong direction?