Welcome to the forum.
The air bubbles generally come from air leaking in to the suction side of the pump, usually between the skimmers and the pump.
Using 3 foot length of 1/2 or 3/4" pipe, hold one end to your ear and the other on the suction piping starting from the pump and going backwards towards the skimmers. You will be able to hear the air bubbles passing through the pipe until they suddenly stop. That is where the leak is.
If you still hear the air bubbles when the pipes go underground, then it becomes much more difficult to isolate.
The most likely place though could come from the skimmers themselves, and you should be able to see air bubbles being pulled into suction line or lines. I think that this is where it is, as you say you lose level 3 to 4 inches overnight.
Keeping the pool crystal clear is by following the "stickies" in the pool operations forums.
With a green pool, see if there is a lot of debris on the bottom, i.e. leaves, twigs, dead frogs etc. These have to be removed with the pool rake and basket. When that is done remove any debris in the skimmer and pool pump baskets.
Start the circulation, and add 4 gallons of 6% bleach, to get the chlorine level up to 15 ppm. You will probably have to backwash the filter a few times, as the pool may get cloudy. If it gets really cloudy, shut the circulation down for 24 hours to let the cloudiness settle, and then vacuum to waste.
Take a pool water sample and post the results if you have a test kit. If not take it to a pool dealer that does not use test strips.
You will want the following test values as guidelines;
CL 15 ppm
pH 7.2 - 7.6
TA 80 - 100 ppm
CYA 40 - 50 ppm
CH 200 - 400 ppm
Now that you have taken over the pool duties, a green pool will take patience and persistence, and eventually you will see a difference.
Hope this helps to get you started, and keep us posted.
Pat
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