The water pressure in the pipe in 150 ft of slope to the top of the dam is way way higher then the pond at 3 ft of depth so it will all push out into the pond on that side leaving your pipe empty, I have done exactly what you are doing and it will work like a million bucks, the only thing you will have to do different is put a check valve in the pond side of the pipe under the water so that the water can flow in and let the siphon work but will hold the water in the pipe when you fill it till you get the tee capped tightly and the valve open or cap removed on the outside down slope side.
Again, the 5 acre pond has almost zero pressure at 3 ft of depth, it could be 105 acres and it only has the pressure of three ft of head. while the pipe going uphill to whatever elevation change there is will give you the calculation of the water pressure at that point, it will run empty as soon as there is three ft of water in your siphon pipe the rest will run empty as fast as you fill it.
It is also imperative to fill the pipe slowly so that all the air can escape as you are filling it so that you do not have any air bubbles in the line anywhere. I have drained many ponds this way without any pump other then to fill the pipe at the tee. Good Luck!
