library(ggplot2)

a1 <- rnorm(mean = 0, n = 200, sd = 1)
a2 <- rnorm(mean = 1, n = 200, sd = 1.2)
a3 <- rnorm(mean = 2, n = 200, sd = 1.4)

b_mean <- sapply(list(a1,a2,a3), mean)

b1 <- density(a1)
b2 <- density(a2)
b3 <- density(a3)

ggplot()+
  geom_path(data = data.frame(X=b1$y, Y=b1$x), aes(x=X, y=Y)) +
  geom_path(data = data.frame(X=b2$y, Y=b2$x), aes(x=X+2, y=Y)) +
  geom_path(data = data.frame(X=b3$y, Y=b3$x), aes(x=X+4, y=Y)) +
  geom_path(data = data.frame(X=c(0,2,4), Y=b_mean), aes(x=X, y=Y))