The average gross monthly household income will be based on both your salaries. Based on your household income, you may get an Enhanced Housing Grant (EHG) of $55,000. You can read more about the schemes and grants via HDB's link here: https://www.hdb.gov.sg/cs/infoweb/residential/buying-a-flat/new/schemes-and-grants/cpf-housing-grants-for-hdb-flats/firsttimer-applicants
Hope this answers your question.
Based on the information given, it will be 3000+(600x12)/12. It may be advantegeous for you to purchase a resale immediately after you start work to enjoy a higher EHG grant but do note your loan quantum you are eligible for might be reduced. I do provide a free and non obligatory consultation in regards to property transactions in Singapore. Do contact me at 93375051 for a discussion on this.
ERA Realty Network Pte Ltd