2001 Nissan Altima Altima Burning Excessive Amounts of Oil

In the past 6 months I have had cylinder work, new brakes (that still squeal), complete air conditioning service including new compressor. Just two years ago I purchased a new catalytic converter. Now my car is burning 2-3 quarts of Penzoil 10W30 a week. The dealership has performed all service and told me that my catalytic converter came loose and has become injested in the engine. I did not have them service the car because the proposed cost was about $6,000. Car was making noise, I checked oil and had NONE so put in 3 quarts myself. I took the car to a small local garage the next morning for an oil change - I am faithful to change oil every 3,000 miles. Mechanic said oil was like black gunk. I only drove 6 miles between putting the oil in and taking the car to the garage. What is going on? Advice?