at，stage前的介词是in

前面的介词是at，例如：

1、We're now in the process of developing the function further， so that at stage three we'll be able to issue invoices.我们现在正在进一步完善该功能，以便在第三阶段能够解决发票问题。

2、People tend to work hard at this stage of life.人在这个人生阶段往往发奋努力。

3、At one stage it looked as though they would win.有一段时间，他们好像大有获胜的希望。