Just wondering since my project includes a few third-party plugins and one I made myself.
If not, how hard would it be to change them so they can be used in C3?
No - C2 addons cannot be used in C3.
This C3 SDK section has the info you'll need to convert from C2 to C3.
You can have third-part Construct 2 add-ons in Construct 3 by asking their authors to port them using our SDK.
Otherwise, check the Exchange Addon or this dedicated forum (and this topic) to see if a port already exists.
Finally you can try this beta converter.
Once you have a Construct 3 version of the add-on, you can install it using the add-ons manager.