Once a month, twice a day, four times a week BUT
How do I say: once TWO years, once IN TWO years? Twice (in) three years?

Question

Answer

The best way to say that would be "Once every two years" "Twice every three years", etc.  You can say, "Once in two years", but it isn't expressing frequency in the present or future like in "once a week", but rather how often something happened in the past.  I hope this helped, let me know if you have any questions!