Google’s (GOOG) Android platform had another winning quarter, according to comScore’s Mobilens. Android phones went from under 15% of the U.S. smartphone market in June to over 21% in September according to a report by ComScore. Google started out the year at 7%. Apple held steady at 24.3%, although July saw a spike with the launch of the iPhone 4.

RIM (RIMM) remained the overall leader, but its market share has dropped from over 40% in May to 37.3 percent in September. While it has a huge installed base, its monthly sales numbers have fallen behind Apple and Google.  The writing is on the wall.

Android has been the best selling smartphone platform for the past several months, outselling phones running iOS and the Blackberry OS. With Google’s current rate of ascent, they are poised to become the number one smartphone platform in the U.S. in the coming year.

That’s if the market stays still. Microsoft (MSFT) and HP (HPQ) have big plans in 2011 for their respective mobile platforms and a Verizon(VZ) iPhone could add some untapped market share for Apple.

Apple (AAPL), which sells other devices with iOS, including AppleTVs, iPads and iPod touch, still has a higher overall sales rate than Google’s Android. Nokia (NOK), the global smartphone leader, which has a strong presence in India and China, doesn’t play a big role in U.S. smartphone sales.