MQ 5:
This gas sensor can detect Butane, propane, methane, hydrogen and other combustible gases.It is capable of detecting various gases.This arduino-compatible special sensor raise alarms when the smoke, methane or any other harmful gas leaks in its reach.
MQ 3:
This alcohol sensor is suitable for detecting alcohol on your breath, just like your common breathalyzer. It has a high sensitivity and fast response time. Sensor provides an analog resistive output based on alcohol concentration. The drive circuit is very simple, all it needs is one resistor. A simple interface could be a 0-3.3V ADC.
Please review the datasheet for conversions to ppm then Wikipedia.org for BAC.
MQ 2:
This sensor monitor many kinds of gas, including: gas, butane, propane, methane, alcohol, hydrogen, smoke, etc. (It is a basic composition of range of MQ 3 and MQ5 )
Read more information on this wiki page.