We’ve said it before, and we’ll say it again. Microsoft is way behind in mobile. But the company, which is banking on the success of Windows Phone 7 and just signed a critical deal with Nokia, could topple at least one of its primary rivals in the next three years. At least that’s the word from Gartner, which projects that Windows Phone will control 19.5 percent of the market by 2015. That compares to 17.2 percent for iOS, the mobile operating system behind the iPhone.

That’s a huge potential leap for Microsoft. Gartner says Microsoft currently owns just 4.2 percent of the smartphone market.

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Google, meanwhile, will continue to grow with estimates that its Android mobile operating system will more than double

It’s similar to another recent prediction by the IDC research firm.

At whose expense are those gains coming? That would be Symbian, with projections its current lead will drop from 37.6 percent today to 0.1 percent in 2015. (Symbian is the mobile OS behind Nokia, which just struck a deal with Microsoft).

From the report:

Gartner predicts that Nokia will push Windows Phone well into the mid-tier of its portfolio by the end of 2012, driving the platform to be the third largest in the worldwide ranking by 2013. Gartner has revised its forecast of Windows Phone’s market share upward, solely by virtue of Microsoft’s alliance with Nokia. Although this is an honorable performance it is considerably less than what Symbian had achieve in the past underlying the upward battle that Nokia has to face.

Meanwhile, All Things D points out that Gartner was actually predicting that Microsoft’s smartphone share would continue to decline just a few months ago. But in fairness to the research firm, that report came out before the deal between Nokia and Microsoft. However, it does speak to how much things can change in the market, which was also the point of a great piece in BetaNews.

They do a little truth-squadding on the Gartner report, pointing out that the smartphone market is “too volatile” to predict what will happen three years out. Reporter Joe Wilcox says he will kiss Steve Ballmer’s feet if Windows Phone moves into the second place slot by 2015.

Now, we’re not rooting for anyone here. But we’d sure like to see that.

John Cook is co-founder of GeekWire. Follow on Twitter: @geekwirenews and Facebook.