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 6,7,8s, iPhone 6, iPhone SE About 44px by 44px
iPhone 6,7,8s Plus, iPhone 6 Plus About 66px by 66px
The difference is because the iPhone 6,7,8[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.