Yes, this should be doable with a reconcile precondition onA where the precondition is checking which resources it needs to wait on based on whetherB orC are created. At least, that's what I would try first.

So the reconcilePrecondition is evaluated multiple times, not once right? I that case there I could check the status, not only the spec

Hmm, you probably shouldn't storeexternal state in the status… Seehttps://javaoperatorsdk.io/docs/dependent-resources#external-state-tracking-dependent-resources (but maybe that's not what you're doing).
To be clearer, it's fine to make decisions based on resources' status (like, for example, if a pod is ready or not).

Oh sure, I mean I could check if dependent resource status has some expected status so the condition is true ir false

So the reconcilePrecondition is evaluated multiple times, not once right? I that case there I could check the status, not only the spec

I mean it will be triggered again (the whole reconiliation) anyway if some of other resources (B,C) changes.