The Falcon Heavy has 27 engines (Suck on it N-1!) in three sets of 9 (per core booster). SpaceX initially suggested they might need a couple of static fires to nail down the ignition sequence, since it turns out to be hard to get that many engines all running in very short time windows.
I had asked in a question about how the engines start whether the burning engines next to each other ignite the following engines or if they use TEA-TEB on all of them. The answer was they use ignitors on all the engines.
Now Falcon Heavy is of course, really running as three seperate boosters, connected. Thus no doubt the same is true.
But if an engine failed to ignite via TEA-TEB would the fact the other engines are rapidly burning ignite the Kerosene and LOX that would be pumping out of the unstarted engine?
I am sure they would shut the stack down when the sensors detect a failed start, but would the missed engine actually start from external ignition sources?