I have 4 covariates - $A$, $B$, $C$ and $D$. Three of them ($A$, $B$, $C$) independently show strong correlation with the forth one ($D$). I can create a chart in Excel for each pair ($A$-$D$, $B$-$D$, $C$-$D$), add a trend line, and get an equation for the trend line. In my case all of them show quadratic trends with the fourth covariate $D$.
I would like to create an equation using the three covariates ($A$,$B$,$C$) to predict the forth one ($D$).