Anomaly detection in multiple parameters
One way to use both metric1 and metric2 in order to find anomalies in metric3 is to consider residual analysis. 

In your case, this would require, creating a predictive model with metric1 and metric2 as the predictors and metric3 as the response variable. 

Then, calculate the residuals for metric3 as its predicted value subtracted from its true value. Now, you can report the all members of the lowest decile [or any other percentile] as one kind of an anomaly and all the members of the highest decile [or any other percentile]   as another kind of an anomaly.