After the Canon Speedlite 430EX III-RT was announced earlier this week, Canon USA revised its press release to note that the flash will work as a Master or Slave unit when using the RT (radio transmitter) mode. In optical mode, it is limited to serving as a Slave unit.
What this means is that you can build out a Canon Speedlite system with just the 430EX III-RT units and no need to buy the more expensive 600EX-RT or ST-E3-RT to serve as the Master controller for your other 430EX III-RT units.
The Canon Speedlite 430EX III-RT retails for $299.99 and is expected to ship in September 2015. More details here at B&H Photo.