Running script in Crontab every 3 days once

The */3 is in the wrong column your specification should be

30 19 */3 * * root /var/scripts/svn_backup.pl 

The */3 syntax is the same as saying 1,4,7 ... 25,28,31. Note that some months it will run on 31^st and the next month it will run on the 1^st because the interval specifications do not wrap, they always start at the beginning of a sequence. In this case your script gets run without the required interval.

If you need something to run every 3 days then you would have to run your script every day and have it determine if 3 days have passed and exit/continue as appropriate.

In perl you could do something like

if ( int (time/86400)%3 != 0)  {exit 0};

which would run the script every 3 days based on the UNIX epoch.

It's wrong. The last column is the day of week. You probably want to move */3 to the third column (day of month):

*     *     *     *     *      command to be executed
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- day of week (0 - 6) (Sunday=0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)