Change case of n-th letter in a string

With GNU sed (possibly others)

sed 's/./\U&/3' <<< "$str"

With awk

awk -vFS= -vOFS= '{$3=toupper($3)}1' <<< "$str"

In bash you could do:

$ str="abcdefgh"
$ foo=${str:2}  # from the 3rd letter to the end
echo ${str:0:2}${foo^} # take the first three letters from str and capitalize the first letter in foo.
abCdefgh

In Perl:

$ perl -ple 's/(?<=..)(.)/uc($1)/e; ' <<<$str
abCdefgh

Or

$ perl -ple 's/(..)(.)/$1.uc($2)/e; ' <<<$str
abCdefgh