Need some 3ft helium balloons

This operation would require several large balloons, and a whole lot of helium.
We blocked our streetlight a couple of years. For us, the streetlight is on our side of the street, with the boom that supports the light hanging out over the street, away from my house.

We used a piece of half inch plywood with one side painted flat black, and used a bicycle cable fastened to the plywood, in an arch on one long side.
We hung the plywood up, suspended by the cable that was fastened over the boom of the streetlight, we put the black side facing the light. The flat black kept the plywood from reflecting any light back into drivers eyes.

Because the shield was placed fairly close to the lightsource, it blocked out a large amount of the light that would normally be projected towards my house.
It cast such a great shadow that quite a few TOTs refused to come up to my door, and because the darkness was so stark and sudden, it made it seem a great deal darker than it actually was. Not hi-tech, but it worked well.

The biggest fight was getting it up and down. The first year we did it, we just used a rope tossed over the boom of the light, and hoisted the board up, then tied the rope off to the vertical pole of the streetlight. It worked, but it made me nervous.
The second year, we used the cable to secure the board, it meant that I had to put up a 16/32 extension ladder on the light pole, then haul the wood up and secure it, a bit scarrier than the rope method, but no worries about the cable wearing through for the night. It was a lot of work taking it up and down, but the effect was very dramatic.

As far as the city letting us do the board bit, I don't think they knew about it, it went up late in the afternoon, and came down early in the morning, and the only place it effected was my house and the sidewalk in front of my house, the street illumination remained the same.

I live next to a military base, and they (the government) won't allow highflying balloons or buildings over two stories for about an eighth of a mile around. They are very paranoid about big balloons, especially ones that can carry stuff, or interfere with flight operations.

Those big weather balloons require a whole lot of helium, and they don't last all that long, so if you are looking at doing your haunt for more than a night, be prepared to spend quite a few dollars for the balloons and helium.
The cooler the air, the more difficult it will be to get your balloons filled, if you are looking at severe temperature changes, then you have to be careful not to overfill the balloons, the gas expands with heat, so what seems fine in one temperature, may be way too much (pop!), or way to little (rasin).