public static class test1
{
    public static ISearchContext Driver
    {
        get
        {
            return webDriver;
        }
    }
}

public static class test2
{
    public static test3 test3
    {
        get
        {
            var Test3 = new test3();
            PageFactory.InitElements(test1.Driver, Test3 );
            return Test3 ;
        }
    }
}

public class test3
{
    public void SwitchToFrame()
    {
        test1.Driver.SwitchTo().Frame(webDriver.FindElement(By.XPath("some xpath")));

    }
}

public static IWebDriver Driver
{
    get
    {
        return webDriver;
    }
}

driver.switchTo().Frame("YourFrameName");