Note that the provided answer assumes your property files address different execution profiles. If they dont, i.e., your properties are split into different files for some other reason, e.g., maintenance purposes, divided by business/functional domain, or any other reason that suits your needs, then, by defining a profile for each such file, you are just "abusing" the profile feature, for achieving your goal (multiple property files per app).
You could then ask "OK, so what is the problem with that?". The problem is that you restrain yourself from various possibilities that you would otherwise have. If you actually want to customize your application configuration by profile you will have to create pseudo, sub, profiles for that since the file name is already a profile. Example:
Your application configuration could be customized by different profiles, which you use inside your springboot application (e.g. in @Profile() annotation), let them be dev, uat, prod. You can boot your application setting different profiles as active, e.g. 'dev' vs 'uat', and get the group of properties that you desire. For your a.properties b.properties and c.properties file, if different file names were supported, you would have a-dev.properties b-dev.properties and c-dev.properties files vs a-uat.properties b-uat.properties and c-uat.properties files for 'dev' and 'uat' profile.
Nevertheless, with the provided solution, you already have defined 3 profiles for each file: appname-a.properties appname-b.properties, and appname-c.properties: a, b, and c. Now imagine you have to create a different profile for each... profile(! it already shows something goes wrong here)! you would end up with a lot of profile permutations (which would get worse as files increase): The files would be appname-a-dev.properties, appname-b-dev.properties, app-c-dev.properties vs appname-a-uat.properties, appname-b-uat.properties, app-c-uat.properties, but the profiles would have been increased from ['dev', ' uat'] to ['a-dev', 'b-dev', 'c-dev', 'a-uat', 'b-uat', 'c-uat'] !!!
Even worse, how are you going to cope with all these profiles inside your code and more specifically your @Profile() annotations? Will you clutter the code space with "artificial" profiles just because you want to add one or two more different property files? It should have been sufficient to define your dev or uat profiles, where applicable, and define somewhere else the applicable property file names (which could then be further supported by profile, without any other configuration action), just as it happens in the externalized properties configuration for individual springboot apps
For argument completeness, I will just add here that if you want to switch to .yml property files one day, with the provided profile-based naming solution, you also loose the ability to define different "yaml document sections per profile" inside the same .yml file (Yes, in .yml you can have one property file yet define multiple logical yml documents inside, which its usually done for customizing the properties for different profiles, while having all related properties in one place). You loose the ability because you have already used the profile in the file name (appname-profile.yml)
I have issued a pull request with a minor fix for spring-cloud-config-server 1.4.x, which allows defining additionally supported file names (appart from "application[-profile]" and "{appname}[-profile]", that are currently supported) by providing a spring.cloud.congif.server.searchNames environment property - analogous to spring.config.name for springboot apps. I hope it gets reviewed and accepted.