[R] How to calculate mean of means

[ecvetano at uwaterloo.ca]

I have 3 columns of numbers, and i want to find the mean of each  
separately, and then the mean of all of the means.
ive tried:

av3daysB<- mean(mean(avr1), mean(avr2), mean(avr3))
av3daysB

but it only gives me the mean of the first column (avr1)

Any suggestions?
Thanks,
Emilija