I had 3 enemy units making a close combat attack against one friendly infantry unit in a surrounded town hex.
Not really. There's no such notion in Memoir '44.
It's not "a" Close Assault being executed by 3 units. It's a series of 3 individual Close Assaults... in fact, with no need for the your enemy to commit to any of them until he'd resolved the prior ones.
Maybe that is the source of your confusion? Because it seems to me that if you look at it that way the Ambush card is unambiguous.