You are basically going to have to test the plugins first - I suggest trying the upgrade on a dev server - you can copy across the plugins and see how they behave. The custom code will need to be added one file at a time - there are huge amount of changes between 2.7 and 3.3 and you might find that your code is either no longer needed or needs rewriting.
Installing and upgrading help
Custom PlugIns and code before upgrade
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