The words may sound similar, but they mean different things.
The registrar is the approved organization that provides the registration service to the public or other companies, for example in Netz0 we use multiple ICANN accredited registrars depending on the extension or pricing. The registrar provides TLD’s extensions like .com, .net, and .org just to name the top ones as well country level domain names, also called ccTLD’s such as .ca for Canada or .de for Germany.
The registrant is the person or organization that registers and manages a domain name on the Internet. For example, a company or an individual. The registrant is the administrative contact and owner of a domain name registration. In Netz0 you can update your domain registration details in real time which is sent to the registrar and then published live on the Internet. The registrar is basically the legal owner of the domain name.
The registry is the legal company and organization that created an extension and manages the infrastructure, so it resolves on the Internet, like DNS resolutions for that particular extension. The registry sells the extensions to the registrar and set the rules that govern the domain like expiration grace periods or transfer rules between providers. For example, VeriSign manages the registration and DNS resolution for all the .com domains names on the planet regardless of where it was registered.