Yo yo!
I have very little technical knowledge so please excuse any senseless mistakes.
I want a phone that I can use that is private and secure for the next couple of years. I understand that new android phones will be made differently so I can’t side load (whatever that means, I just know it is important). I have an iPhone 12 rn but I’d rather leave the iOS ecosystem. Rn I’m typing this message on a pixel 3.
Is a pixel 3 old enough to still be a safe and secure phone that I can side load apps on in the future?
As others have said, look into an alternate OS as your best option. I only have experience with GrapheneOS, but it’s super simple to install and functions pretty well out of the box.
Unfortunately your pixel 3 is too old and no longer supported. 6 is the oldest supported and they recommend 8 or newer because those have 7 year support cycles as opposed to 5 on earlier models.
That aligns with your wants though, as a pixel 6 is 4 years into its support cycle so oud only get another year out of it. Maybe you’d be OK with a 7 and 2 years, but if you went 8 then you’d have 5 years remaining. So it depends on how future proof you want vs your budget. General recommendation would be to buy a phone second hand.
Take a look at their FAQ for more info. It’ll probably answer a lot of your questions. https://grapheneos.org/faq
I’ll also say that even if you run GOS in a non private manner, its still more private than a standard android phone. I know somebody who runs GOS with google services on (gasp!) but that is still more private because it runs sandboxed and not with full reign of the OS. GOS also has a lot of options like turning your mic on and off and disabling Bluetooth. Even simple things like that are good first steps in getting into a private phone.