Lock file and release it
Solution 1:
Using flock is probably the easiest way for three competing shell scripts. Let's assume you have a single path to a lock file called /run/lock/myapp.
# in terminal 1
flock /run/lock/myapp sleep 10
# in terminal 2
flock /run/lock/myapp echo "hello"
If you run these at about the same time (the first first though) the second will wait 10 seconds before running.
If you're deep in another programming language, there will be other ways to handle this better.