We are already seeing the decline of 3G coverage as investment is focused on 4G and other newer technologies. The 3G networks' days are numbered, with Telstra announcing a shutdown of their 3G network in June 2024.
The below Telstra coverage maps show a dramatic difference in coverage today and is just one example as of 1st March 2019. If you have a 4GX device, you will have good service. If you have a 3G-only device in Sydney or the Blue Mountains you would experience frequent black spots and areas of weak signal incapable of providing a reliable PTT (data) connection over 3G.
There are also plenty of rural areas where there is only 4GX (Band28) coverage - so if you don't have a device covering the required Telstra frequencies, ie. all of Band 28, not just half of Band28; or just the higher 4G frequencies - like many competitors' devices - you will have no coverage at all in these areas.
Make sure you have fully tested your devices in the areas you operate in, before you buy them.
Example 1: Sydney 4GX vs 3G coverage (https://www.telstra.com.au/coverage-networks/our-coverage, 3 March 2019)
Even with an external antenna, 3G coverage today still cannot even come close to the blanket coverage of 4GX:
Example 2: Lower Blue Mountains 4GX vs 3G coverage (https://www.telstra.com.au/coverage-networks/our-coverage, 1 March 2019)