If you don't know the row number, but do know some values then you can use subset
x <- structure(list(A = c(5, 3.5, 3.25, 4.25, 1.5 ),
B = c(4.25, 4, 4, 4.5, 4.5 ),
C = c(4.5, 2.5, 4, 2.25, 3 )
),
.Names = c("A", "B", "C"),
class = "data.frame",
row.names = c(NA, -5L)
)
subset(x, A ==5 & B==4.25 & C==4.5)