There are some thoughts on how to go about the upgrading process in the expressionengine documentation - and elsewhere in stackexchange. Start with these sources, and if you have problems beyond what is covered through such information, post specific questions back here and I am sure there are many who can help.
Based on the list of add-ons you have provided, you should be able to upgrade fairly easily.
- Export Entries, Freeform Pro, Field Pack and Tagger all have EE3 support, so simply a case of paying for the upgrades as required.
- Matrix and Playa have an upgrade path to EE3, but Grid and Relationships now offer these functions from within EE, and there is a good / free upgrade utility available from EllisLabs. Worth making the effort to swap over to native EE function here I think.
- Mailing List(eeharbor) - not sure what this is (doesn't appear to be discussed on eeharbor site anyhow) - so maybe something extra will be needed to do for this one
As noted in the two linked posts to upgrade to EE3 you first have to update the EE2 version of site to a late version of EE2 (at least 2.10). Worth updating all your add-ons to final versions for EE2 at same time, and would make sense to do the Playa / Matrix migration to native EE before moving to EE3 too. Only once you have a strong updated EE2 platform in place should you start the EE3 upgrade process.
You didn't say why you are only looking to upgrade to EE3 - the upgrade from EE3 to EE4 is really much less complex than the one from EE2 to EE3, and brings with it some really useful new features - such as the just launched GDPR toolkit which makes building and operating a GDPR compliant site very much easier. Might be worth thinking about going the extra few yards to get EE4 in place since you are doing an upgrade anyway.