how to put a red frame before each video programmatically, when concatenating videos using ffmpeg

To merge files with ffmpeg I'm creating a text file like this:

file 'a.mkv'
file 'b.mkv'
file 'c.mkv'

Then I'm running this command to concat these videos:

ffmpeg -f concat -i file.txt -c copy merged.mkv

But the thing is, I want to put red frame for three seconds in the middle of each video.

Now I can get these videos' frame dimensions and create a video and modify the text file like this:

file 'red.mkv'
file 'a.mkv'
file 'red.mkv'
file 'b.mkv'
file 'red.mkv'
file 'c.mkv'

But this is not a programmatical approach, so is there any way I can concat videos and put red frames for three seconds (before every video)?

I want to generate that red frame video, on run time.

bash script:

#!/bin/bash
intro=a.mkv
main=b.mkv
outro=c.mkv
red=red.mkv
VID=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=codec_name -of default=nw=1:nk=1 "$main")
WID=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=width -of default=nw=1:nk=1 "$main")
HEI=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=height -of default=nw=1:nk=1 "$main")
SAR=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=sample_aspect_ratio -of default=nw=1:nk=1 "$main")
if [ "$SAR" = "N/A" ]; then SAR=1; fi
FPS=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=r_frame_rate -of default=nw=1:nk=1 "$main")
# if video has variable framerate, you can get something like 22840/769
FPS=30
TBN=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=time_base -of default=nw=1:nk=1 "$main")
TBN=${TBN#*/}
FMT=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=pix_fmt -of default=nw=1:nk=1 "$main")
echo $VID $WID $HEI $SAR $FPS $TBN $FMT
AUD=$(ffprobe -v 0 -select_streams a:0 -show_entries stream=codec_name -of default=nw=1:nk=1 "$main")
CHL=$(ffprobe -v 0 -select_streams a:0 -show_entries stream=channel_layout -of default=nw=1:nk=1 "$main")
SRA=$(ffprobe -v 0 -select_streams a:0 -show_entries stream=sample_rate -of default=nw=1:nk=1 "$main")
echo $AUD $CHL $SRA
ffmpeg \
-f lavfi -i "color=c=red:s=${WID}x${HEI}:r=${FPS}:d=3" \
-f lavfi -i "anullsrc=cl=$CHL:r=$SRA" \
-c:v $VID -c:a $AUD -video_track_timescale $TBN -shortest $red -y
echo --- "$red"
ffprobe -v 0 -select_streams v:0 -show_entries stream=codec_name,width,height,sample_aspect_ratio,r_frame_rate,avg_frame_rate,time_base,pix_fmt -of csv=print_section=0 "$red"
ffprobe -v 0 -select_streams a:0 -show_entries stream=codec_name,sample_rate,channels -of csv=print_section=0 "$red"
l="list.txt"
echo "file '$intro'" > $l
for f in $main $outro; do
[[ ! -f $f ]] && continue
echo "file '$red'" >> $l
echo "file '$f'" >> $l
done
cat $l
ffmpeg -f concat -i list.txt -c copy output.mp4 -y
mpv output.mp4

maybe, you have to add something to this code