Sending bash script variables to sed

first time poster here and I searched for quite some time but didn't find a thread I could understand well enough to solve my problem.

I have a loop in my script that generates certain variables via cut -d delimitation and I wan to use that variables as input for sed to alter a file differntly each iteration. My current script is:

My current script looks like:

for input in $(tail -n 3 filea)
do
        a="$(echo $input | cut -d "," -f 1)"
        b="$(echo $input | cut -d "," -f 2)"
        c="$(echo $input | cut -d "," -f 3)"

        echo $a
        echo $b
        echo $c

        tail -n 1 fileb | sed 's,'$a','$c',2'
done

With an output of:

K
26
T
KTTXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
P
27
E
KKTXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
S
31
P
KKTXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Where for example the string on position 31 should be replaced with S if it is P.

My current status is that I get the variables a and c correctly transfered into the sed command and respective strings get replaced but changing it to sed 's,'$a','$c','$b'' stops any replacement and the native (last) line of fileb gets prompted.

I know that asking a good question is a whole other topic, but I hope somebody can help me to understand the writing here so I can continue to learn programming as I often step over mistakes like these.

Notes:

• don't use for for reading lines of a file
• use IFS and read to split a line using a separator
• be more rigourous about quoting variables.

Given:

$ cat filea
K,26,T
P,27,E
S,31,P

$ cat fileb
-------------------------TE---P---

then

tail -n 3 filea | while IFS=, read -r replacement position character; do
    printf -v regex '^(.{%d})%s' $((position - 1)) "$character"
    printf -v repl  '\\1%s' "$replacement"

    echo "$regex"
    echo "$repl"

    tail -n 1 fileb
    tail -n 1 fileb | sed -E "s/$regex/$repl/"
done

outputs:

^(.{25})T
\1K
-------------------------TE---P---
-------------------------KE---P---
^(.{26})E
\1P
-------------------------TE---P---
-------------------------TP---P---
^(.{30})P
\1S
-------------------------TE---P---
-------------------------TE---S---

But, this is something that bash can do without sed, using the ${var:offset:length} form of Parameter Expansion:

tail -n 3 filea | while IFS=, read -r replacement position character; do
    echo "$replacement   $position   $character"
    line=$(tail -n 1 fileb)
    echo "$line"
    if [[ "${line:position - 1:1}" == "$character" ]]; then
        line=${line:0:position - 1}${replacement}${line:position}
    fi
    echo "$line"
done

outputs

K   26   T
-------------------------TE---P---
-------------------------KE---P---
P   27   E
-------------------------TE---P---
-------------------------TP---P---
S   31   P
-------------------------TE---P---
-------------------------TE---S---