Update dotnetcore.yml

.github/workflows/dotnetcore.yml

@@ -5,14 +5,18 @@ on: [push]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macOS-latest, ubuntu-latest, windows-latest]
 
     steps:
     - uses: actions/checkout@v1
     - name: Setup .NET Core
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v1.2.0
       with:
-        dotnet-version: 3.0
+        dotnet-version: 3.0.100
+      if: matrix.os == 'macOS-latest' || matrix.os == 'ubuntu-latest'
     - name: Restore Dependencies
       run: dotnet restore
     - name: Build