This methods given below will work for almost all HTC devices.

To enter into Bootloader mode, do this:
  • Turn off your phone
  • Hold down Volume Down + Power button
  • Release the Power button when the device starts and continue to hold the Volume Down key until you see a white screen, that’s the bootloader.
To enter into Fastboot mode, do this:
  • Turn off your phone
  • Hold down Volume Down + Power button
  • Release the Power button when the device starts and continue to hold the Volume Down key until you enter into the bootloader. [For some phones, bootloader mode itself acts as Fastboot mode]
  • Scroll up or down with Volume keys and select Fastboot with the power key.
To enter into Recovery mode, do this:
  • Turn off your phone
  • Hold down Volume Down + Power button [older devices: Home + Power button]
  • Release the Power button when the device starts and continue to hold the Volume Down key until you enter into the bootloader.
  • Scroll up or down with Volume keys and select Recovery with the power key.
Exclusions:
HTC Legend

To enter  into Bootloader/Fastboot: 

  1. (while turned off)Press and hold Down on Trackball + Volume Down + Power To enter into Recovery
  2. (while turned off) Press Down on Trackball + Volume Down + Power 
  3. Highlight Recovery with the volume keys and select with the Power key.