An Australian Monarchists spokesman said the league was delighted by the announcement and sent their "warm greetings and congratulations" to the royal couple.
The Duchess gave birth to Princess Charlotte of Cambridge just two years ago, where she also suffered from Hyperemesis Gravidarum, which causes severe morning sickness. Prince George is set to begin his first day of school on Thursday, September 7, at Thomas's Battersea in south London.
No details were immediately available about when the third baby is due, but betting agencies were quick to start offering odds on possible names for the soon-to-be-born child in the House of Windsor.
William and Kate were married in April 2011.
On a royal tour of Poland in July 2017, Kate joked about having a third - after being given a present designed for newborns.
There were several signs that Kate could be expecting, according to reports.
Prince Charles, Queen Elizabeth II's oldest son, is now No. 1 in the succession line.