As I stated before a few drops of oil does not account for the loss of 3.0l per 1500Km
If you were leaking this much oil the entire undercarrige of you car would be dripping wet and the exhaust would be covered with oil makeing a very bad burnt oil odor.
As far as diag. of this is concerned The first order of buisnes is to remove the sparkplugs and inspect for the pressence of oil residue on the plugs. Then procede with a compression test and a cylinder leakage test. the CLT is much more accurate than the compression test. If nothing is discovered with these test then have the clyinders inspected with a bore scope. It is possible that all you need is valve guide seals. If the compression test and the CLT show up negitive and there is oil residue on the spark plugs I would replace these seals.