temp
[1] "2014/12/22" "14:48:51"   "800"

temp <- gsub("/","-",paste(temp[1],paste(temp[2],temp[3],sep="."),sep=" "))
[1] "2014-12-22 14:48:51.800"

op <- options(digits.secs = 3)
options(op)
Date_time <- as.POSIXlt(strptime(temp, format="%Y-%m-%d %H:%M:%OS"))
[1] "2014-12-22 14:48:51.8 IST"