Take values from a colum if other two columns are matched

tidyverse
3

Is this what you want to do?

library(tidyverse)

df %>%
    filter(Col.2 > 750, Col.2 < 849) %>% 
    arrange(time) %>% 
    mutate(Col.6 = if_else(Col.2 == Col.4, Col.3, NA_real_)) %>%
    group_by(time) %>% 
    fill(Col.6, .direction = "down")

If this doesn't solve your problem, please provide a minimal REPRoducible EXample (reprex) illustrating your issue.