percentage of variance explained by factors

Hello!

I've run a simple linear model: result<-lm(DV~A+B+C,data=Data)

My Data$A,Data$B, and Data$C are factors. So, lm automatically recoded
them into dummy variables. I have all the results I need but one.

Question: Where could I see the variance explained by all A dummy
variables together, then all B dummy variables together, and all C
dummy variables together - when other predictors are present (in other
words, the sum of squared semi-partial correlations for all A dummy
variables, B dummy variables, and C dummy variables)? Or is it just
not possible with lm?

Thank you very much!

Dimitri Liakhovitski
MarketTools, Inc.
Dimitri.Liakhovitski at markettools.com