HoraDecimal <- function(x){
  sum(unlist(lapply(strsplit(x, split=":"), as.numeric)) * c(1, 1/60, 1/3600))
}

x <-  "02:20:00"
HoraDecimal(x)
[1] 2.333333

y <-  "00:50:00"
HoraDecimal(y)
[1] 0.8333333

z <- "3:30:30"
HoraDecimal(z)
[1] 3.508333