gas chamber for calibrating gas sensors
To be accurate you will need standard gas cylinder and pneumatic connections (pressure regulator, needle valves or better mass flow controllers, pipes). You will need a measuring chamber with an overflow. As a rule of thumb, if the chamber geometry is not exotic (lets say a cube with no dramatic dead volume), you need to flush it, at least  10 times, with each concentration you want to measure.

You can do a 2 points calibration for O2 by measuring atmosphere where O2 concentration can be assumed constant (span) and then flush with pure nitrogen (zero). For CO2 and CO you will need standard cylinders.

Anyway a 2 points calibration is a little bit risky because you will not be able to assess linearity and then you will have no idea in which extent your sensor is linear (dynamic range). Properly calibrating sensor requires multiple points calibration and therefore a dilution device and standards.

Caution: When dealing with gas you must take care of exhaust and prevents risk such as intoxication (especially CO and in a fewer extent CO2) or asphyxiation (due to a lack of oxygen) and explosion (CO is explosive).

You should find those materials, resources and skills in a University (Chemistry or Physics Department) or an Air Quality Laboratory.

Note: If your sensors are electrochemical ones, you need also to assess their correlations/deviations with temperature and humidity.