I see DevOps as a culture shift that is working towards a culture where developers can easily build, deploy, monitor and alert on their code in production. It is a direct counter-attack to the pattern of developers throwing their code over the wall and hoping that a sys admin will deploy it. You need expertise in tooling and processes in order to achieve the culture shift, that is where these 3 positions come in.

I see each role as an evolution where you go from DevOps -> SRE -> Platform Engineer. Automating the infrastructure and tooling is the catalyst. You can have any of these roles in your company based on how far along the path your company is, how on board management is and how interested your development teams are in growing/learning these practices.

DevOps writes scripts/code to automate CI/CD pipelines, creation of infrastructure, deployments. Once automated you move onto..

SRE writes scripts/code monitoring, alerting, SLOs/SLAs, incident processes. Once automated you move onto..

Platform Engineering where you write the tooling to make all of the above codified. Tooling could be anything from cli tools to kubernetes operators.

I say this as someone that has spent the last 3 years automating my way towards platform engineering.