Arranging rows in custom order using dplyr

We can use factor to change the order in a custom way

df %>%
    arrange(factor(Reg, levels = LETTERS[c(3, 1, 2)]), desc(Res), desc(Pop))
#  Reg   Res    Pop
#1   C Urban 501638
#2   C Rural 499274
#3   A Urban 500414
#4   A Rural 500501
#5   B Urban 499922
#6   B Rural 500016

Or with match to get the index and arrange on it

df %>%
    arrange(match(Reg, c("C", "A", "B")), desc(Res), desc(Pop))

If we have multiple columns to arrange in descending order

df %>%
     arrange_at(2:3, desc) %>%
     arrange(match(Reg, c("C", "A", "B")))