Where is ERC20Mintable.sol in OpenZeppelin Contracts 3.0?

Support Contracts
2

OpenZeppelin Contracts v3.0 has a beta release

The final v3.0 release is not yet finished

Roles contracts (such as MinterRole and PauserRole ) were removed: we’re redesigning our Access Control solution and will have a better version of these in the v3.0 release.

ERC20Mintable.sol is part of OpenZeppelin Contracts v2.5 which uses Solidity 0.5.
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.5.0/contracts/token/ERC20/ERC20Mintable.sol