Here are two approaches if you want to remove the trailing 0 characters and convert to POSIX. The first uses base r, the second with tidyverse packages.
x <- c("17/04/20 17:48:04,000000000", "17/04/20 21:47:12,000000000", "15/04/20 22:19:43,000000000")
strptime(gsub("\\,[0-9]*", "", x), "%d/%m/%y %H:%M:%S", tz = "UTC")
#> [1] "2020-04-17 17:48:04 UTC" "2020-04-17 21:47:12 UTC"
#> [3] "2020-04-15 22:19:43 UTC"
lubridate::dmy_hms(stringr::str_remove(x, "\\,[0-9]*"))
#> [1] "2020-04-17 17:48:04 UTC" "2020-04-17 21:47:12 UTC"
#> [3] "2020-04-15 22:19:43 UTC"
Created on 2020-05-01 by the reprex package (v0.3.0)