03 Bash

S=0
# read the input file line by line
while read line; do
    # get the first half of the string
    x1=${line:0:${#line}/2}
    # second half
    x2=${line:${#line}/2:${#line}}
    # iterate over characters in the left half
    for (( i=0; i<${#x1}; i++ )); do
        # that is the character at position $i
        ch=${x1:$i:1}
        # if $ch contained in the string $x2?
        if [[ "$x2" == *"$ch"* ]]; then
            # convert chars to their ASCII values
            n=$(printf '%d' "'$ch")
            # a = 97 b = 98 ... A = 65 B = 66 ...
            if ((n < 96)); then
                # turn values 65... into values 27...
                a=$((n-64+26))
                S=$((S+a))
            else
                # turn values 97... into values 1...
                a=$((n-96))
                S=$((S+a))
            fi
            break
        fi
    done
done < "03.input"
echo "$S"

#!/bin/bash
# shebang is highly recommended

set -e # fail immediately

S=0   # sum
N=1   # line number
L1="" # next-to-previous line
L2="" # previous line
L3="" # current line

while read -r line; do
    # update last three lines
    L1=$L2
    L2=$L3
    L3=$line
    # every 3rd line
    if ((N%3==0)); then
        # iterate over characters in $line string
        for (( i=0; i<${#line}; i++ )); do
            # get the $i-th character
            ch=${line:$i:1}
            # if $ch is both in the prev and the before-prev line
            if [[ "$L2" == *"$ch"* && "$L1" == *"$ch"* ]]; then
                # get the value with the ASCII trick
                n=$(printf '%d' "'$ch")
                if ((n < 96)); then
                    a=$((n-64+26))
                    S=$((S+a))
                else
                    a=$((n-96))
                    S=$((S+a))
                fi
                break
            fi
        done
    fi
    ((N++))
done < "03.input"
# print the result
echo "$S"