At the top of your script you could put:
NUM_INSTANCES=$(ps aux | grep simu | wc -l)
if [[ $NUM_INSTANCES -ge 4 ]]; then
echo "3 simu instances already running..."
# the rest of your code here...
Here we check if your script is found 4 times in the process table rather than three, because "ps aux | grep simu | wc -l" is counted itself there. As written this function will check every 5 seconds, but you can change that by modifying the "sleep" value. Once there are less than 3 instances running the rest of your script will run...
There is a possibility for false-positives here (if the string "simu" is found elswhere in your process table) but this will dissappear if you choose a more unique name for your script.