I would do the same as you suggested: schedule a script to run every day and check the date, and if day of the month is equal to 3 then proceed with whatever you wanted to do.
So, you need to declare variable for a day of the month in order to proceed. For example:
:local date [/system clock get date]
:local day [:pick $date 4 6]
:if ($day = "03") do={
BODY OF YOUR SCRIPT
}