echo A12B3456CD78 | sed "s/\([0-9]*\)$/\\
\1/" | sed "1 s/\([a-zA-Z]*\)$/\\
\1/"

[/tmp]$ cat ./try.ksh
#! /bin/ksh

in=A12B3456CD78

f3=$(echo $in | tr '[A-Z]' ' ')
f3=${f3##* }
in=${in%$f3}
f2=$(echo $in | tr '[0-9]' ' ')
f2=${f2##* }
f1=${in%$f2}
echo "$f1"
echo "$f2"
echo "$f3"

[/tmp]$ ./try.ksh
A12B3456
CD
78
[/tmp]$

# !/opt/third-party/bin/zsh
                                                                                 
str="A12B3456CD78"
                                                                                 
only_num=$(echo $str | tr '[A-Za-z]' ' ' | awk '{print $NF}')
only_alpha=$(echo $str | tr '[0-9]' ' ' | awk '{print $NF}')
echo $str $only_num $only_alpha | awk '{ print (substr ($0,0,length($1) - ( length($2) + length($3) ))), "\n", $2, "\n", $3 }'
                                                                                 
exit 0