[/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]$