The message is quite clear, I imagine you are only sending the three screenshots that ask you at least, but these must be very similar and are not showing all the potential and content of your app.
I can understand that there are screens that you can not display for purposes of use, privacy or whatever, or maybe your app is simple and does not have many screens.
Whatever the case, my recommendations are as follows:
1.- If you have more screens, where you show information, personal data or something similar, try to modify this capture with "mockup" data, prototype data that has no meaning for anyone, I mean?
2.- Improve your captures, Apple likes that the content is attractive to the eye, then try to convert the simple to the complex, for example, you can use some tool that generates quite attractive screenshots like Screenshot Builder (There are many more, or if you can create more elaborate screenshots, maybe showing an iPhone prototype or something, what better)
3.- Although Apple asks that the screenshots be of a real device and not the simulator, apps with simulator screenshots are still accepted, mainly if you do not have a specific device to test it. If you are adjusting sizes for screenshots sizes, do not do that, better use the simulator.
4.- In payment apps it is proven that the apple review, especially for the screenshots is much more thorough, Apple is the seller of the product to the customer, therefore the responsible for purchases when the customers are dissatisfied, that is why the screenshots should work as a clear and concise example of what it contains and the functionality of your application.
Sometimes during the publication process we can catch the rush, but this is the most important part of the development of the app. It must be taken with calm and patience to do it well.
