Assuming what can be disconnect is USB ones only.
(Means, I'm assuming you cannot disconnect /dev/ada0 but can disconnect the USB drive containing zroot3.)
If you can boot from zroot2, boot from there with USB drive containing zroot3 disconnected, then, import zroot3 (on /dev/ada0p4) with -R option specifying alternate root (for example, /mnt) and reguid zroot3.
if you cannot, boot from, for example, installation image (live system) on USB memstick. All others should be the same as boot-from-zroot2 example.
You should be better renaming zroot3 which is reguid'ed not to conflict by name with zroot3 on USB drive. After that, USB drive containing zroot3 can be attached again.