low oil

my Mazda 6 was serviced at mecor on13/07/2015.which was a oil change. begging December 2015 I started to hear a engine noise. I took it to a couple of mechanics and it was actually a mechanic at mecor that said I had a bearing knock on my engine. he asked if the car ran low on oil. I said yes . 2 days prier I had a mechanic at a fitment centre checked my oil and found it was very low and very dirty. he replaced the oil and oil filter and told me there was only about 2L of oil in the car when he drained it. he looked for leaks and did other checks and even asked me if the car was smoking or has it used oil in the past. I said no. I have never had to top up on oil in between services. the only explanation is that the oil was never replaced. what happened to about 3l of oil that disappeared out my car. I had the same mechanic look further and check my spark plugs that only supposed to of done about 20000km. these spark plugs are good for 90000 km. I was told that these plugs were not in good shape and he suggests to contact mecor for them to explain and check. I preferred to go the way to keep this public and place my complaint on hello peter. don't trust petrol attendants to check oi