Que tal:
while read line col1 col2 col3;
do
if [[ "$line" = "pos" ]]; then
echo "pos COL"
continue
fi
echo "$line COL1 $col1"
echo "$line COL2 $col2"
echo "$line COL3 $col3"
done < real2.txt
saída:
pos COL
18691441 COL1 C
18691441 COL2 A
18691441 COL3 G
18691572 COL1 G
18691572 COL2 C
18691572 COL3 G
18691620 COL1 A
18691620 COL2 T
18691620 COL3 G
18691716 COL1 C
18691716 COL2 G
18691716 COL3 C