Yes, it drops your status from Full Time to Part Time.
You will loose your rights to concession transport fares, but you only have to pay the SSAF at a part time load. However, if you do a summer course (ie. A semester full time, a semester part time, and a unit over summer), your status for the year becomes full time and you pay full time student SSAF)
Also means you'll spend longer at uni.
3 subjects still counts as full time, though I believe if your an international student you are required to take 4.