Good article. Very comprehensive. Lots of things could be the issue.
Here's another article that talks about the 3 red flashing lights down farther in the article.
Doing some more internet surfing:
Number of Flashes Diagnostic Information
1 w/ 3 sec. pause Air Flow / Limit Fault
2 w/ 3 sec. pause Flame Sense Fault
3 w/ 3 sec. pause Ignition Lockout Fault
As power is applied to the circuit board, the system does the
a. Timing circuits allow the blower to purge the heat
chamber for 15 seconds.
b. When current is supplied to the gas valve it opens to
high burn stage. (The controller module activates the
low burn operation on the valve.)
c. As the valve opens, the ignition module sends a high
voltage spark to the electrode at the burner. The
ignition module detects the presence of a flame. If the
flame is not sensed after 7 seconds of sparking a signal
is sent to the controller module that there is no ignition
and shuts off the valve. After another 24 second purge,
it will try again. After a third try, the controller will go into
“soft” lockout, timing for one hour and the diagnostic
LED will flash a code, See Chart. After the timed hour,
the controller will initiate (3) more tries for ignition. If
there is no ignition, the timing sequence begins again.
d. If the system does not ignite and the thermostat is still
calling for a heat demand, the blower will for 90
seconds as a post purge then shut off.