In TripleA the aa-guns do fire immediately at the Aircrafts (during cm-move).
This is not 100% ruleconform, because with TripleA you can adjust your combat-move to the dice-rolls from the overflown aa-guns, iE.:
you can play like this motto:
oh, all my planes did survived the aa-gun fire, so I can take two tanks less to my attack, or
oh, my fighter or bomber was shot down by aa-gun, so I do not attack the target-Space with any other untis, ...
from the game - rules (LHTR):
In the combat move phase, whenever an air unit
moves over or into a hostile territory
containing an antiaircraft gun, the
antiaircraft gun gets a chance to shoot it
down during the conduct combat phase.
The air unit’s controller plots its path using
the numbered punch out markers to indicate
each space in which combat may occur (the
first space is 1, the second 2, and so on).
Whenever an air unit encounters enemy
antiaircraft guns, resolve combat separately
in each space along its path, starting with
the first antiaircraft gun encountered.