Build a cron job running first weekday of every 3 months

I want to develop a cron job in Ubuntu using R with it cronR package. That job would run on the first Sunday of every 3 months (this 3 should be a variable). However the cronR package does not give any such functionality (as far as I know).

Can anyone give some pointer if it is possible to achieve this using cronR package, or by any other means?

Thanks,