You can add task Use .NET Core and insert needed version in Version field like this 3.0.100-preview9-014004, mark Include Preview Versions as checked.

Execute this task before any other .NET Core tasks.

You can specify .net core SDK version with the .NET Core Tool Installer Task. But I dont think it allows 3.0 yet apparently it does, so you probably need to create a container with 3.0 and use that as a build environment. That would work.

https://docs.microsoft.com/en-us/azure/devops/pipelines/process/container-phases?view=azure-devops&tabs=yaml

sample yaml:

resources:
  containers:
  - container: inflation
    image: yyy
    endpoint: xxx

jobs:
- job: inflate_infrastructure
  container: inflation << has to match the container name above
  pool:
    vmImage: 'Ubuntu-16.04'
  steps:
  - zzz

I use .net core 2.2 but, it released .net core 3.0 preview 6 yesterday. you can use config below. I suggest inspecting release-metadata serves.

steps:
- task: UseDotNet@2
  displayName: 'Use .NET Core sdk'
  inputs:
    packageType: sdk
    version: 3.0.0-preview5
    installationPath: $(Agent.ToolsDirectory)/dotnet

You can install the .Net core SDK 3.0 during the pipeline with .Net Core SDK Installer task:

- task: UseDotNet@2
  displayName: 'Install .net core 3.0 (preview)'
  inputs:
    packageType: sdk
    version: '3.0.100-preview6-012264'
    installationPath: $(Agent.ToolsDirectory)/dotnet

I specified the last preview version of .net core sdk 3.0, you can put an earlier version, you can find here the versions list.

Another option is to specify 3.0.x and enable the preview versions:

version: 3.0.x
includePreviewVersions: true

Installation results:

You can also install the SDK via the "old" Pipeline editor. Add a new task and search for the ".NET Core SDK Installer." Then switch the version at the top to the latest preview version and it will offer you more options, like the "Include Preview Versions" check box. You'll still have to type in the version you want manually.

In case someone is using Web GUI instead of yaml files. It's possible to set up Agent Specification with windows-2019 value.

Such steps as dotnet test will start using .Net Core 3.0:

You can get the latest .NET Core 3 SDK by supplying 3.x instead of a specific version. Also note includePreviewVersions: true, which is required to make this work.

- task: UseDotNet@2
  displayName: 'Use dotnet sdk 3.x'
  inputs:
    version: 3.x
    includePreviewVersions: true

And you'll end up with something like this:

Source: .NET Core Tool Installer task