The sizes for custom navigation bar icons are listed in the iOS Human Interface Guidelines here: iOS Human Interface Guidelines — Custom Icons
The sizes are listed as:
iPad Pro, iPad, iPad mini About 44px by 44px
iPhone 6s, iPhone 6, iPhone SE About 44px by 44px
iPhone 6s Plus, iPhone 6 Plus About 66px by 66px
The difference is because the iPhone 6[s] Plus uses the @3x resources while the others use the @2x images. So your @2x and @3x sizes are correct, your @1x image should however be half the size of your @2x image.
From iOS Human Interface Guidelines — Image Size and Resolution:
A standard resolution image has a scale factor of 1.0 and is referred to as an @1x image. High resolution images have a scale factor of 2.0 or 3.0 and are referred to as @2x and @3x images. Suppose you have a standard resolution @1x image that’s 100px by 100px, for example. The @2x version of this image would be 200px by 200px. The @3x version would be 300px by 300px.
Now, there aren't many iDevices left that use @1x images that you're likely to support so it may not practically be an issue, but it's something you should be aware of.