I would guess if necessary we would pour in up to 10,000 troops and bring in more planes and helicopters if Kabul is really at risk of falling. 3,000 troops are not enough.
Get out of there! Also, and more importantly don’t bring them here.
Having given up the territory, we would be "re-invading" it, and the basic rules of warfare are that an invasion force must be huge relative to the size of the forces they are attacking. You are correct, even if just to hold Kabul and environs, we would be talking tens of thousands of troops.
I am not sure they could get 10,000 troops in there in time.