twrp-3.2.3-0-oneplus3.img has finally solved the decryption problem many of us had for all Firmware/Modem after OB31 :good:
I just updated TWRP, then "Stable5.0.4 Firmware+Modem_OnePlus3.zip" (from OB31), and everything seems fine now.
It is not shown on the list yet, but the file is there:
Yes, just skip the password step and go into "Read-Only" mode. This is enough for ADB Sideload.