While there are other ways we've reported on how to get S-OFF on your HTC EVO 3D, they involve temporarily bricking your phone and reverting back to hboot 1.40 in order to subsequently root with Revolutionary. Unlimited.io released a method over the weekend that does not require a downgrade in order to achieve S-OFF, and will leave you with a 1.50 S-OFF EVO 3D with no need to use other exploits.
The new method will require using the wire trick, a method in which you connect a contact to a ground on the back of the EVO 3D in order to throw the phone into the proper state to flash a modified hboot. You can use some speaker wire or a paperclip from what I've read.
The standard warnings for S-OFF/rooting should be paid attention to. It's really early on in the release of this method for the EVO 3D, you might want to hold off and make sure there's no subsequent issues before you dive into it. There's also no S-ON method listed, which probably isn't a problem considering Sprint's current policies, but might be down the road.
If you do this, let us know how it worked for you. I'll most likely be jumping into S-OFF world again with this method later in the week.[unlimited.io]