SCCM 2012 R2 CU5 Single Primary Site Environment with 1 MP and DP.

When we PXE boot machine how does SCCM select the boot image?

We have 3 boot images and SCCM selects only One Specific boot image everytime. There are multiple task sequences with other boot images as well however it seems to be using only 1 boot image all the time.

How does SCCM decide which boot image to use with PXE Boot?

Why 3 boot images at all?
The boot image that's being used by the tasksequence with the most recent advertismentid will be used. The client will download another boot image if it does not match the one that's configured in the task sequence, reboot and boot that one. 

See http://blog.configmgrftw.com/boot-image-selection-configmgr/ for the specifics.

However, as Torten questioned, why do you have multiple boot images in the first place? Each task sequence in no way requires its own boot image. Standard practice is most environments is to simply maintain two: one for x86 and one for x64.