I just suffered the same,
Google searches sends to this post first, so replying here in case someone else faces this issue.
So far I had found that
-- All the pi 3B plus come with usb boot enabled from factory, not the case for 3B wich supports it but require a change it the config.txt to enable it , so if the slot is already damaged you are out of luck (there seems to be a revision v1.2 of raspberry 2B that also supports boot from usb )
-- On pi zero there are Probe pads that allows to solder a microSD to fullSize SD adapter
PP14 - SD CLK - SD card clock
PP15 - SD CMD -
PP16 - SD DAT0 -
PP17 - SD DAT1 -
PP18 - SD DAT2 -
PP19 - SD CD -
, but those pads are not present on Raspberry Pi3B
With that in mind I proceeded to actually solder the microSD to fullSD adapter to the microSD tracks still alive on my raspberry pi 3B (once again if you have the plus model you do not need to do anything, and you can boot from the USB right away)
The task is simple , everything matches one to one from the microsd paths to the SD with the exception of the 6th teeth wich happens to be ground too, and found that the adapter had them bridged anyway.
Now for soldering it,required thin wires, so cut an old IDE connector for taking the 8 needed wires, it was neither easy nor pretty, but got it done. And then my pi was booting back again from SD,
As a lesson learnt i enabled the usb boot on it, and on the other 3 pi that allowed it, my Pi2 did not work for usb boot as it is revision 1.1