MQ-3 sensor from Sparkfun
In this case, it seems to me your schematic shown is completely wrong,
but I find the MQ3 datasheet to be almost indecipherable. 2 A's, 2 B's,
and 2 H's all being totally different signals. Who draws these things?
Gakk. I find the Sparkfun BOB schematic to also be bizarre.
I wouldn't be confident unless I had the part in front of me, so I could
take a DMM to it, and figure out what comes out what pin.
It looks like from the BOB shcematic that the 5V and Gnd pins are pretty
clear. However, what is the difference between the 2 A's?
In any case, it looks like at least 1 of the A outputs is "analog", so you
need to connect it to an Arduino A/D channel, not a digital channel.
It also looks like you need a sensing resistor to gnd, connected to one of
A outputs, but the MQ3 d/s calls it RS in one place and RL in the other,
so everything is very confusing.
And I don't see why you have those 100, 220 and 470 R's at all.
Good luck.