...it would only serve 300x600 once, and the other two will be 300x250.
It's generally not a good idea to define ads at these sizes, especially if your ads are located "above the fold", meaning in the section of the page a user can see before scrolling. Additionally, you will need to have a lot of content on your page to even qualify for such setup and have the ads spaced apart from each other nicely and not have one right next to another. It's not a great idea to have an ad push content down the page, especially if what users see are ads to start, requiring them to scroll to see content.
Also, take this into consideration. My screen resolution is 800x600 and any of your ads will take up just over 1/3 of my screen width and an ad will take up either my entire screen height or almost 1/2 my screen height. Putting all three ads above the fold and making the site public will cause me to wonder what kind of site I landed on.
I suggest using Google analytics or some other internet software that collects the screen width and height of all devices that connect to your website and determine ad sizes based on the lowest screen resolution.
Also, if the amount of content you have is too little, then go with just only one ad unit, not three.
And as per google's terms, only use responsive ad units if your site is 100% responsive (meaning the same URL can be browsed nicely on a phone and on a standard computer).