Whatever your overarching goal is, you need to know how to measure it, and more important, actually be able to measure it (e.g., have the right digital marketing tools in place to do so). How you measure the effectiveness of your digital strategy will be different for each business and dependent on your goal(s), but it's vital to ensure you're able to do so, as it's these metrics which will help you adjust your strategy in the future.
Yoast - a very useful and FREE (or freemium) WordPress plugin which analyses page copy in a traffic light signal basis for how SEO friendly the copy is based on keyword density, alt-attributes, SEO titles and Meta descriptions, slug etc. It cannot guarantee ranking, as it in no way affects search engine algorithms, but gives a great estimate of how well your copy is written based on your keyword. This is great to use as a guided SEO editor whilst you write and makes you aware of key on-page SEO techniques you should be utilizing.
That's exactly how the three can work together to help you meet your goal. Of course, it's not compulsory to use all three. If your owned and earned media are both successful, you might not need to invest in paid. It's all about evaluating the best solution to meet your goal, and then incorporating the channels that work best for your business into your digital marketing strategy.