Sprint phones are CDMA, so unlocking isn't exactly the same as with a GSM phone (AT&T, T-Mobile).
If you want to use the Sprint HTC EVO 3D phone with another carrier it can be brought to some Sprint MVNOs (Mobile Virtual Network Operator) like Ting as Brian mentioned. Other ones like Boost or Virgin Mobile might be able to active the HTC EVO 3D, but the list of approved devices can vary between carriers.