[R] Three-way ANOVA?

[e.rapsomaniki@mail.cryst.bbk.ac.uk]

Hi,

I'm a biologist, so please forgive me if my question sounds absurd! I have 3
parameters x1, x2, x3 and a response variable y.The sample size is 75. I tried
to do the following:
mylm<-lm(y~ x1 + x2 + x3, data="mydata")

but i can only get stats from anova for the first 2 variables. The third comes
up as NA. The degrees of freedom for the third variable are 0.

Is there something else i could do to fit a model that contains all these three
variables?

Many Thanks
Eleni