Hello!
An iPhone SE, like most modern phones (and not just iPhones), are usually expected to last little more than a day as it’s assumed we are all able to give them a daily charge. Lasting three days made you an exception.
That being said, there are some things you can do to help improve your battery life considering what you said seems to be taking most of it. First of all, go to settings, swipe all the way to the top until you see the search bar appear at the top (above your name). Type in “background” - the list will auto-fill to show “background app refresh”, tap this. You can then simply turn off the ability of the offending apps to do things in the background and save your battery life - in general, apps will largely work in the same way, if you are unhappy you can always turn background app refresh back on.
Also, if you go to settings > battery > battery health, there will be information surrounding the total capacity of your battery as a percentage of when it was new, if this is below 80, Apple would consider that a consumed battery and would likely recommend replacing it. Book your device in at the Genius Bar to do that (make sure you know your Apple ID and password). Rechargeable batteries, like tyres on a car, are just consumable parts and are designed to last roughly around 2 years or a bit longer. If you’ve had the phone longer than this, again, chances are it may need a replacement battery.
