Here is quick guide on how to get the HTC Evo 3D to boot into HBOOT. This was created over at phandroid, as such we take no responsiblity or credit for the following.
1) Check to make sure the fast booting option is disabled. This is located at Settings > Power > Fast boot (uncheck the box)
2) Charge your phone (not essential, but the LED will come in handy if you’re not used to this procedure)
3) Power off the phone by holding the power button. Wait for the popup menu to appear, then tap ‘Power off.”
4) Here’s where the charging LED comes in handy. It will go off. Wait for it to come back on. When it does, the phone is fully shut down.
5) Press POWER AND VOL-DOWN and hold both down. Doesn’t matter what order.
6) after 1 second (or longer), release only the POWER button. The charge light should go off.
7) Continue holding VOL-DOWN until the white HBOOT screen appears. If you let go of VOL-DOWN too early, nothing will happen. Go back to step 5 and repeat.
On the Evo, HBOOT will come up if you just keep both buttons depressed. Now, you need to let go of power button and continue holding VOL-DOWN. Now that you know roughly how long it takes for the phone to react, you can do the same thing without the phone charging.
If you don’t plan to access HBOOT for a while, I would recommend turning Fast Boot back on: Settings > Power > Fast boot (check the box). It really does save a lot of time in the bootup process.