Are @3x images mandatory?

If your application supports the device running iOS 7, then it will not get rejected. One of my application was recently approved (a month ago), which doesn't contain any @3x images.

But it needs to be confirmed for the application that developed in iOS 8.

Edit

For your information:

My application was already on AppStore and It was an update.

Found from What's new on iOS 8

Support for a New Screen Scale The iPhone 6 Plus uses a new Retina HD display with a screen scale of 3.0. To provide the best possible experience on these devices, including new artwork designed for this screen scale. In Xcode 6, asset catalogs can include images at 1x, 2x, and 3x sizes; simply add the new image assets and iOS will choose the correct assets when running on an iPhone 6 Plus. The image loading behavior in iOS also recognizes an @3x suffix.

I fear that you need to provide @3x images for all asset images if you are going to publish your application for iOS 8.0 and for iPhone 6 plus device. As it clearly says that "The image loading behavior in iOS also recognizes an @3x suffix".

I suggest you include @3x images before going for approval.

No, the @3x suffix is only recommended if you're supporting iPhone 6+ (iOS 8+) although not required at the current time. Below are the requirements and recommendations for image sizes.

Icon and Image Sizes