This page has a list of what's new in C3 over C2.
Multiplayer is using the same technology and making multiplayer games is a complicated and costly task no matter what engine you are using.
You can open C2 capx in Construct 3 as long as you have the correct addons in Construct 3 to open your project.
Standards addons are working out of the box.
You can have third-part Construct 2 add-ons in Construct 3 by asking their authors to port them using the SDK.
Otherwise, check the Exchange Addon or this dedicated forum 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.
As a owner of a C2 license visit:
https://www.construct.net/profile/my-discounts
The page contains informations on how to apply the discount to your subscription.
In my usage and opinion C3 is worth it (would it be for the search function alone that has been a life-saver to me).
The answer to this question depends from people to people though, according to what you are looking to do and your situation.