I've already payed for Minecraft pocket and Xbox one edition, do I really have to pay for it again on Windows?

Ok

Well, you do. It's very rare to get any game on a different platform regardless of if you own it already.

Yes, you do. If you want the game for a different system, then you'll have to pay for it again, it doesn't matter how many versions or copies of the game you own.

You will need to purchase the PC version SEPARATELY as each version you purchased already (Pocket Edition & XBox One Edition) were purchased through different marketplaces (Mobile Edition via Google Play or iTunes Store, XBox One via XBL Marketplace) that are beyond Mojang's control.

I will note that by purchasing the PC version, you do get BOTH the standard Java edition AND the Windows 10 edition (that can connect to the other versions). Please be aware that the Windows 10 edition must be purchased through the Windows 10 store with the gift code provided through Mojang's website. The Windows 10 edition will be PERMANENTLY TIED to your Microsoft account upon redemption, so make sure you're logged into your account when redeeming your code.