In a company I'm working we are planning to get some devices in order to test our software on it. We are doing web app, so I need to know Is there a big difference between Windows 8 IE, Windows phone IE and Windows RT IE In other words should we test on all three devices or testing on desktop IE should be enough?
As far as I know, the Internet Explorer found on Windows Phone uses the same engine as the desktop version of IE. I think the only difference between them is the viewport/screen sizes that they'll be viewed on.
I think really you can test mobile sites on the desktop version of IE the same way you would with Safari, just resize the screen.
According to this article: https://www.smashingmagazine.com/2015/05/testing-for-windows-phone/
As Microsoft acknowledged in the past, the version of Internet Explorer found on mobile devices is similar but not identical to the desktop browser version. There are a few key features that are not available on Windows Phone: HTML5 touch drag-and-drop support (demo). Videos which use the Encrypted Media Extension will not play on Windows Phone. CSS Touch Views other than overflow:scroll CSS3 hyphenation is also not available These are all rather minor features but you still need to know about the rendering differences and take these into account.