Error on Craft 3 install
In order to install under MAMP with PHP7 we added the MAMP PHP7 path with the following command:


  export PATH="/Applications/MAMP/bin/php/php7.0.15/bin:$PATH"


After achieving a successful Craft 3 install we ran the setup in the CLI and we also created a fresh DB and a new MAMP host following the sequence recommended in the installation docs.