I'm going to a Macquarie Megaplex in Sydney,Australia and i think that its either monday or tuesday.
And if i watch it on the cheaper day will watching it in 3D be cheaper as well?
I don't know about your specific theatre, but I can tell you about my experience with theatres that offer discount days.
The first theatre I worked for had a "Tightwad Tuesday". All movies which were more than a week old would be discounted all day long (at the time I think it was $2.50). That theatre still has the discount day, and it does apply to 3D movies. However, you will still be charged the 3D premium above the regular ticket price. For example, if a 3D movie normally costs $3.00 more than a regular movie, the 3D will likely cost $3.00 more than the discount day price.