Virtually all Construct 2 content using official features should port in to Construct 3 with no problems at all. We've worked hard to ensure there is excellent backwards compatibility right the way back to the earliest versions of Construct 2.
If you have trouble importing content, usually it's because it depends on third-party addons that were not ported to C3. I'm afraid these addons are the responsibility of their respective developers, so you'll need to contact them for support.
Remember that any time you rely on third-party developers, you may need them for help and support for years to come. You may wish to research reliable third-party developers who are willing to provide long-term support - and note since that involves potentially a lot of work over time, you may need to pay them for that.