Probably you need cron-jobs
https://en.wikipedia.org/wiki/Cron
Server will execute scheduler based on time you define in your cron-job
, In Your case it will be good to schedule it every day.
now every day it will fire you scheduler > then it will call your Task
(https://octobercms.com/docs/plugin/scheduling#introduction) with your logic.
Logic
You will fetch records with condition like :
current_date + 1/2/3 day
convert that date in to timestamp and store to variable future_date
And add condition on field expiry_date
expiry_date > today_time_stamp AND expiry_date < future_date
It will give you records which are about to expire in next 1/2/3 days
now notify [send email]
or fire event
for these records
You can add master cron job to your server just follow this guide
https://octobercms.com/docs/setup/installation#crontab-setup
Then you can just write you logic how you want thing to be done
https://octobercms.com/docs/plugin/scheduling#introduction
if any doubt please comment