Frys Electronics HSBC Retail Services I'm being fleeced by HSBC,Mitsubishi and Frys Concord, California

Over 3 years ago now I bought a big screen Mitsubishi TV at Frys Electronics in Concord,CA. It was on a special sale @ 1500 + tax (I was happy.) Then I followed up with an extended payment plan for another $300 (which included an extended service contract). This extended the no interest payment plan from 3 years to 5 years (and the same for the service contract. (I was very happy.) I was making my payments going along getting closer to having my TV paid off when I got a notice that my promotional payment plan would be ending. I called Frys,Mitsubishi and HSBC to find out what was going on and was told that there was no record of the payment plan being extended. I produced my paperwork and sent it to all parties involved but got no response from anyone. Then went down to Frys and there was no record of any extended payment plan(evidently my sales person did not have the authority to do such a deal). I asked Frys,Mitsubishi and HSBC to take responsibility for this situation and back me up but got no response from anyone. My next bill arrived and I was charged over $1700 in back interest(more than the TV was worth) and started a new payment program at 21% interest per month. I was very upset and made copies of all the paperwork I had and sent it to all parties involved - I've done this 3 times now over the past 18 months and the situation is not resolved. I've now paid over $3000 for my $1500 TV and will have paid over $5000 by the time I get it paid off. This is robbery and is criminal and I should not have to pay anymore on this TV and in fact,my be due for some compensation on this whole thing. Any help I can get from anyone would be greatly appreciated. Best, hector,Walnut Creek,California