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Edit1:

Edit2: Approach 2

Edit3: Conclusion

##Approach 2

##Conclusion

Edit3: Conclusion

1. There should ideally be no out parameter instead an aggregate class should be evaluated in Asserts.
2. If there are unrelated out parameters then SUT method should be refactored into multiple methods, then those new methods should have their own Test methods and they will then test only one concern.

If redesigning of SUT is not possible then as per the answer there will be three Asserts

Edit2: Approach 2

    public void DosSomething_Passing15_Returns12Test(){
        //add code for arrange
        int expectedvalue=12;
        string expectedVariable1="I am expected";
        string expectedVariable3=15.9;
        string actualVariable1, actualVariable3;
        var sut=new Foo();
        int actualValue=sut.DoSomething(15, out actualVariable1, out actualVariable3);
        
        Assert.IsTrue(actualValue, expectedvalue);
    }
    
    public void DosSomething_Passing15_ReturnsExpectedVariable1Test(){
        //add code for arrange
        int expectedvalue=12;
        string expectedVariable1="I am expected";
        string expectedVariable3=15.9;
        string actualVariable1, actualVariable3;
        var sut=new Foo();
        int actualValue=sut.DoSomething(15, out actualVariable1, out actualVariable3);
        
        Assert.IsTrue(actualVariable1, expectedVariable1);
    }
    
    public void DosSomething_Passing15_ReturnsExpectedVariable2Test(){
        //add code for arrange
        int expectedvalue=12;
        string expectedVariable1="I am expected";
        string expectedVariable3=15.9;
        string actualVariable1, actualVariable3;
        var sut=new Foo();
        int actualValue=sut.DoSomething(15, out actualVariable1, out actualVariable3);
        
        Assert.IsTrue(actualVariable3, expectedVariable3);
    }

Isn't approach 2 adhering to "Tests should not test more than one concern"?