library(dplyr)
library(matrixStats)
nm1 <- c('hp', 'drat', 'wt')
res1 <- mtcars %>% 
           mutate(Mean= rowMeans(.[nm1]), stdev=rowSds(as.matrix(.[nm1])))

head(res1,3)
#   mpg cyl disp  hp drat    wt  qsec vs am gear carb     Mean    stdev
#1 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4 38.84000 61.62969
#2 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4 38.92500 61.55489
#3 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1 33.05667 51.91809