I like @Javier answer (which I actually used in my code, so thanks!). I needed to perform further work on that, and I found a quick and dirty way of doing the same

data <- data.frame(
entry = paste0("Entry.", 1:10),
"A" = c(0,0,0,0,1,0,1,1,0,0),
"B" = c(1,0,0,0,1,1,1,1,1,0),
"C" = c(1,1,1,1,0,0,1,0,1,1))
aggregate(data$Entry,by=list(data$A,data$B,data$C),FUN="length")