How to configure HCI UART for 3 Mbps?

HCI/SPP refer to the communication protocol. They both use a standard UART, just at different baud rates. However, the SPP mode and HCI mode are very different. HCI mode bypasses a large part of the Bluetooth software stack, so you would have to implement that yourself somewhere if you want to use HCI mode. To change the mode, you have to upload new firmware onto the bluetooth module. This is detailed in the datasheet.