In my opinion, in alternative 1 and considering your particular case of 3 structures, you must consider also the different seismic displacements of group supports belong to different structures.
I mean even the supports can be at the same level but in different structures, it is likely they will be subject to different imposed displacements under a seismic event.
In fact for a structure and a level the supports are subject to a spectrum considered in phase for those supports, but for two structures and a level, you have two groups of supports, subject to different spectra which are out of phase in the worst case. It is what is named in literature Seismic Anchor Movement- SAM.
That's why I think the alternative 2 is better, even is hard in practice and time consuming.