I concur with HVAC1000 on this. As stated earlier, at this juncture, getting a experienced Tech in there is best and safest.
There is way more to this than you are thinking and the reply I gave is in your best interest.
Just for your knowledge, myself and HVAC have a combined 68 years between us in the HVAC Service/Repair/Install trade.
You are guessing at problem and it is dangerous enough with Nat. Gas, but way more so with Propane.
This is for your safety and your families.
If they could not resolve it in 3 trips, either they are not qualified, or the furnace is in need of replacement.
You will need someone on site with experience with that unit.
Take care and best wishes,
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?