Unit doesn't heat. Is blinking three times.how do I test the end switch?

"end switch"? Pressure switch perhaps? What does the flash code say?

How to Troubleshoot the Goodman Furnace 3 Blinks | eHow

I can only assume you checked the vents and grills first, and wiring, and hoses.

Goodman furnace pressure switch | WebHVAC.com

Here are a few good videos to test a pressure switch

https://www.youtube.com/watch?v=t_dsSOdPqXw

Or

https://www.youtube.com/watch?v=qQ2b-DZ82Bs