Given that a portfolio consists of Stock=USD 30, High-yield bonds(duration=5 years,spread duration=5 years) =USD 40 , Commodity = USD 30.
You actually don't need to worry about correlations. Just calculate the portfolio value over time using your fixed weights
[.3, .4, .3].
The advantage of this approach is;
- You don't have to worry about correlations.
- You can use any volatility method you want (rolling window, GARCH, etc).