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.
All three sentences are colloquially correct when spoken, and totally unambiguous. B would sound more formal because of the words "has been", but otherwise their meanings are all the same.
However, C is the only one which is correct in writing. The phrase "since I have done this…" is (I think) only properly correct when the word "since" is interpreted as "because", as in "Since I've eaten this pie, I'm not hungry any more". It sounds a bit clumsy even in this context.
Personally, in writing, I would prefer to mix B and C:
It has been two weeks since I (last) saw you.
And in speech, I would contract "it has":
It's been two weeks since I (last) saw you.
In speech and informal writing, the word "last" could optionally go at the end instead, but it reads a bit oddly in very formal writing:
It's been two weeks since I saw you last.
Best Answer
The construction would be
have + past participlein such cases.Here, "I saw..." would depict a distant past opposite to '..have seen' which is a recent past.
Both construction would work depending on what you want to say.