Which sci-fi book am I thinking of? Tripods?

[william]

We read it in middle school a long time ago. It is about a group of people who escaped from the alien "Tripods" that walked around in giant 3-legged machines and the people are trying to find shelter or something. The tripods know where they are constantly because of metal-plated tracking devices planted under the peoples' armpits. There is one part where a character cuts of the tracking devices in a gruesomely painful scene.

Anyone know what book this is?

 

[Henry]

The tripods series. The three books were: the white mountains, the city of gold and lead, and the pool of fire. There was also the prequel, when the tripods came. There by john christopher