This is called backfire.This backfire can occur in air intake,air filter and in exhaust as well eventually car loose power for a while.
It is related to some of the reasons like,improper tuning and setting for intake valves which might be opening too soon than they should,this phenomenon is called timing advance,fuel is not burning real time.
Improperly manoeuvred air/fuel ratio by the 3s might be another cause,it might occur due to leaning, like fuel is insufficient and air is entering abundantly again because of loose air filter box,still it occurs if fuel is more than enough in combustion chamber,as i said earlier improper tuning.
If they have advanced the timing,plug will fire the spark before the intake valve completely seats at cylinder head,this way the ignited fuel will find the way out towards inlet,finally explosion will occur in intake manifold,this explosion will come out through air filter box,most of the time burning air filter with it,so check your air filter keenly if this is burnt or have any sign of burning,replace it at once(at least 300/= rps ka patakha).
Even on vehicle which are not well tuned,when ever one change the gear, i mean shifts up,one will leave the clutch.At this moment engine finds a moment of rich fuel which is not burnt properly will cause a backfire in exhaust .
Idle air control is a sensor attached on the air filter box,if this sensor is malfunctioning,backfire may occurs,to test is go for scan test.
Bad plugs,plug leads and coils are also some of the causes of backfire but mostly not.(our desi mechanics always try to be over smart telling you coils are out).
Fuel filter if clogged and fuel pump if malfunctioning can cause disturbance in air/fuel ratio,so can become a cause of backfire.
So my advise is to go back to 3s and discuss with them on these issues.