usually the observed deterioration (called degregation) is much less than the expected or guaranteed degregation. Manufacturers usually guarantee for at least 80% after 20 years. Of course the panel needs to be directed in the right ankle to the sun and the panel needs to be free of dust and shadow. Especially shadow over the panel - even if it is a small one - can be very bad for efficiency. On the other hand you have to observe in summer time at full sunlight.
Depending on where on this globe you have built your PV plant, 3 kw out of a max of 5 kw is actually pretty good for this time of the year...
If it is not getting better in full sunlight there might be a problem with the panel and the panel manufactuerers guarantee for the output as described above...
but very good that you log the data, so you can observe on your own and you got proof.