Both FCOM 2 and 3 have a lot of useful information in them.
FCOM 2 is required for all your contaminated PERF. It also contains various procedures that you may need to refer to, for example flight with the gear down etc and lots more!
FCOM 3 has lots of good stuff too. Limitations, expanded checklists, SUPP techniques, useful for manual engine starts, cross bleed starts just to name a couple.
From an operational point of view, as you become more experienced as an airbus pilot you will learn your way round the manuals.