In a practical sense, yes you can do that and it will work in all the mostly used architectures and compilers.
See "Typical alignment of C structs on x86" section on Wikipedia.
More details:
floats are 4 bytes and no padding will be inserted (in practically all cases).
also most compilers have the option to specify the packing of structures and you can enforce that no padding is inserted( i.e. #pragma pack from visual studio )
arrays are guarantied to be contiguous in memory.
Can you guarantee that it will work in all CPUs in the world with all the compilers? No.. but I would definitely like to see a platform where this fails :)
EDIT: adding static_assert(sizeof(A) == 3*sizeof(float)) will make this code not compile if there are padding bytes. So then you'll be sure it works when it compiles.
