Learn English – “has not seen” versus “has not been seen”

The first two are examples of present perfect passive. The verbs are respectively "has been sent" and "have been registered". We do not know who did it, that is why we use the passive. Sent and registered are the past participles of the verbs send and register.

The third sentence is an example of present perfect active, where the verb is "have been" (worried is an adjective). You can put it in any tense you want, depending on the context, for example "I was worried about you but now I am fine." or "I have never been more worried in my life."

If you expand the second example further, you will see that worried can not be the verb in this sentence: "I have never been (quite) as worried about you as in these past few hours."

Typically we use the past simple (saw) for events in the past and the past perfect (had seen) for events further in the past. For example, you could say:

I had seen my brother asleep when I left home in the morning, but then later I saw him at the bank.

You'll need to give us more context for these sentences to know which one would be more correct, because they can both be used in different situations.