With 3 gallons of deadspace. Say you have a 13lb grain bill and you want to mash at 1.25qt\lb of grain. You would need 4.06 G of strike water. Add the 3 G and you're at a little over 7 G. Did you mash in at 180 or was that the temperature after you got all stirred in. You denature all enzymes at 172. If your mash was 180 then that would explain for no conversion and your lack of activity in your airlock.