1993 Ford Taurus tell me how to change the three back spark

Since you are asking this question, I would say that you have the 3.8 engine. The plugs are accessible with a swivel spark plug socket and an extension. They are a bit tight, but can be reached. The heater tube that goes along the back of the engine is generally the main interference point. You can loosen the retaining nuts at the top of the manifold, and push it somewhat out of the way for better access. But, I didn't say it would be easy. Also, be careful starting the replacement plugs, as the 3.8 has aluminum heads, and crossthreading is easy.