Company A holds my h1b visa, contracted to Company B tier 1 vendor and the end Client is Company C.
Company C offers me a permanent position after working for three years. Company B agrees to relieve my duties. Company A comes back and say I cannot join Company C because it is his end client.

There was some statement on the offer letter of compamny A stating I cannot join company A customers or clients and the liquidated damazes was $15,000.00 amount.

Please suggest

Look into the possibility of getting a new visa which would allow you to work for Company C.

" If a foreign worker in H-1B status quits or is dismissed from the sponsoring employer, the worker can apply for a change of status to another non-immigrant status ..." http://en.wikipedia.org/wiki/H1B_visa