Engine Mechanical problem
2000 Buick Lesabre 6 cyl Front Wheel Drive Automatic 120000 miles

why does my engine start and run good, but then starts to run rough and dies? and then it starts hard and runs really rough and then dies again, and acts like it is running on only 3 cyls. I have changed all 3 coils and spark plug wires and there is no difference and the service engine light stays on constantly.

The first thing to do is read the codes, Autozone does it free, if you cant make it there, buy an OBDII scantool by following this link, retieve and post all codes!
http://www.2carpros.com/affiliate/uid/merlin2021_1