It si not clear what you mean by "I have 3 rows of data" but here is an example of how to subset a named vector.
row3 <- c("A" = 1, "B" = 2, "C" = 3)
row3
#> A B C
#> 1 2 3
mean(row3[c("A", "B")])
#> [1] 1.5
sd(row3[c("A", "B")])
#> [1] 0.7071068
Created on 2019-09-15 by the reprex package (v0.3.0.9000)
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