C++ does give you the power to lay your data members how you would like. But in this case you've just said that you want those three members to be laid out sequentially in memory. If you want to use column storage because you need to apply a specific transformation on the data then lay it out that way.

In the vast majority of cases having classes be automatically laid out in column based storage would be a detriment and not an advantage. You would actually be fighting against the cache in those cases.

For example I've seen rendering engines go from storing data in AoS to SoA, then back, then back again, then back again all depending on the hardware and tech stack available.