There is a lot to be said about each of these eCommerce platforms. While Amazon and eBay drive traffic and make sales for you, you don't get to build your own brand. Shopify allows you to create a comparable website to Amazon and eBay but doesn't take a huge fee and you can keep all your customer's data and build an email list, plus build a brand.
I recommend starting with Shopify, then choosing to scale by selling on multiple marketplaces like Amazon, eBay, or even Houzz.
You can get a free unlimited trial of Shopify by signing up for my high-ticket drop shipping course here: https://academy.ecommerceparadise.com
Free 99 niches list: https://ecommerceparadise.com/99dropshipniches
Free mini-course: https://ecommerceparadise.com/dropshipsecrets