How to get PPM value from MQ-3?
A series of if() statements and some simple division should be able to provide you with your PPM results.

From the datasheet, Alcohol Gas Sensor（Model：MQ-3), figure 5 shows the sensitivity curve:



It can be seen here



that there are three regions that the curve can be broken into:


The relationship is linearly proportional between 2.25&nbsp;V and 3.25&nbsp;V
Between 3.25&nbsp;V and 3.8&nbsp;V there is a curve
The relationship is linearly proportional between 3.8&nbsp;V to 4.25&nbsp;V


The slope of the straight line regions described in points 1 and 3 can be represented by the use of a constant and the PPM value by a simple division of the voltage by the constant. 

The curve from the second part could be:


approximated by a straight line (thus, once again, using a constant), for simplicity, or;
more accurately, using a lookup table.


Using a straight line approximation for the curve (shown in lurid green), the graph becomes:



To determine the constant for the three lines simply take two points on each line and use:


  (y2 - y1) / (x2 - x1)


So for region one, assuming (by approximation, 3&nbsp;V corresponds to 100&nbsp;ppm and 2.5&nbsp;V corresponds to 85&nbsp;ppm (my eye sight may be slightly off here), the slope is

(3 - 2.5)/(100-80) = 0.5/20 = 0.025


You can calculate the constants for regions two and three in a similar manner (if using a straight line approximation for region two).

Therefore the code would be alomg the lines of

const double kRegionOne = 0.025;   // region one constant
const double kRegionTwo = ???;     // region two constant
const double kRegionThree = ???;   // region three constant

int ppm;                           // ppm result

/*
 * your code here
 * ...
 */

if ((sensor_volt &lt; 3.25) &amp;&amp; (sensor_volt &gt;= 2.25)) {
  ppm = sensor_volt/kRegionOne;    // we are in region 1
} else if ((sensor_volt &lt; 3.8) &amp;&amp; (sensor_volt &gt;= 3.25)) {
  ppm = sensor_volt/kRegionTwo;    // we are in region 2
} else if (sensor_volt &gt;= 3.8) {
  ppm = sensor_volt/kRegionThree;  // we are in region 3
}


If you wish to be more accurate then use a lookup table for region two, as jsotola states in their comment.