Try this:
#!/bin/bash
if [ "$1" == "--on" ]; then
python3 script.py --option a & # Note: Fixed typo 'pyhton3' to 'python3'
python3 script.py --option b &
elif [ "$1" == "--off" ]; then
pkill -SIGINT -f "python3 script.py --option a"
pkill -SIGINT -f "python3 script.py --option b"
fi