You can do this if you define custom assembly configurations in the solution file.
Open the solution in Visual Studio, then open Configuration Manager for the solution. The Active Solutions drop-down list will have the <New...> option, which will allow you to create x86 and x64 platforms for the solution. After you create them, select each of them in the same drop-down list and make sure that you have Any CPU for your projects A and B in the list and that project C has x86 or x64, respectively. Also make sure the Build checkbox is selected.
Now switch the Active solution configuration to Release and define these additional platforms in the same way.
After that, you can build all 3 projects that define only the solution file on the MSbuild command line:
msbuild ABC.sln /target:Build /property:Configuration=Debug;Platform=x86 /verbosity:minimal msbuild ABC.sln /target:Build /property:Configuration=Release;Platform=x86 /verbosity:minimal msbuild ABC.sln /target:Build /property:Configuration=Debug;Platform=x64 /verbosity:minimal msbuild ABC.sln /target:Build /property:Configuration=Release;Platform=x64 /verbosity:minimal
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