Depends on the model, or actually technically it depends on the bootcode but which version is preloaded depends on the model.
The brand new Pi 3B+ ships with the latest bootcode and will boot from USB mass storage or PXE by default. Their PXE is a bit nonstandard but it's close enough that anyone who's PXE booted a PC will understand it.
The older Pi 3 supports USB and PXE booting, but has those modes disabled by default. There's a bit you can set in one-time programmable memory to enable it permanently. PXE booting has some quirks in this mode and doesn't get along with switches that take more than a second or so to activate ports.
Earlier models do not support anything but SD on their stock bootcode.
In all cases, a SD card containing nothing but the latest bootcode can be inserted which brings the new features to the older models, allowing them to boot the rest of the OS from whatever.
At least for the older models you still needed the card, but only for a marker that tells the Pi to boot from USB. I've been using that setup with Kodi/OSMC with a Pi 2 since USB is faster than the SD card.
The brand new Pi 3B+ ships with the latest bootcode and will boot from USB mass storage or PXE by default. Their PXE is a bit nonstandard but it's close enough that anyone who's PXE booted a PC will understand it.
The older Pi 3 supports USB and PXE booting, but has those modes disabled by default. There's a bit you can set in one-time programmable memory to enable it permanently. PXE booting has some quirks in this mode and doesn't get along with switches that take more than a second or so to activate ports.
Earlier models do not support anything but SD on their stock bootcode.
In all cases, a SD card containing nothing but the latest bootcode can be inserted which brings the new features to the older models, allowing them to boot the rest of the OS from whatever.