According to a Yuanta Securities report, Apple already expressed its interest in integrating rear tri-camera setups for its upcoming high-end smartphones. The future iPhones could get 5X optical zoom per lens and it looks like there will be at least a 12 MP sensor. Apple's tri-cam devices are slated to be released in 2019.
https://www.notebookcheck.net/Triple-rear-camera-iPhones-coming-soon.302886.0.html