# Bash 4+, 104 bytes

Replaces first char of the first (non-empty) input string with 1,2,3 until no match is found to either input

* *saved 11 bytes thanks to ideas from GammaFunction*

```
[ ${#1} = 0 ]&&shift;
x="$1"
for((;i++<4;)){
o=$i${x:1}
[[ $o = $1 ]]||[[ $o = $2 ]]||{ echo $o;exit;}
}
```
[Try it Online!][1]


  [1]: https://tio.run/##PYu7CoQwEAD7/YrFW3xg44qVe/kSSXGKklQBtQjEfPteuOKagRmY9XM51QUpvTijwQFtXV/OH7dANBVxBUc421Z8378n6boEwZCnFGfOsJQxlIsYrX2ev44/TbhvLpQke/S3ZMiq2jCKcsEX