The north London boroughs follow Lambeth Council who also joined the 2M Group in the last two weeks.
2M represents more than 2 million people in communities affected by aircraft noise.
The alliance was originally formed in 2006 to protect the quality of life for residents under the Heathrow flightpath.
The group is not anti-airport but argues that further growth is unsustainable.
Consultation on the Government's plans for a third runway has now closed. Transport secretary Ruth Kelly will announce her decision in the summer.
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The proposals would boost capacity from 480,000 to 702,000 movements a year. This is equivalent to building a new airport the size of Gatwick.
In the short term ministers want to introduce mixed mode operations on the two existing runways. This would bring an end to the current practice of runway alternation which provides noise relief at 3pm each day.
Hounslow Council's Leader, Peter Thompson, speaking on behalf of the 2M Group said: "The fact that three new councils have come on board in such a short space of time shows the scale of opposition to the Government's plans is building.
"We started out with the name 2M because this was the combined population of the boroughs. The next council that joins will almost certainly take us through the three million mark. This will send a powerful message to the government that millions of people say 'no' to Heathrow expansion."
(CD/JM)
