Since this is labeled lateral-thinking, I'll give it a shot. The answer is pretty simple really:
Two of the ants are walking backwards.
While walking backwards, words like "front" and "behind" become ambiguous. When you are walking backwards in some direction, someone ahead of you can be referred to as being in front of you, but they are also literally behind your back so behind you. And vice versa.
Ok, that's interesting, but does it add up?
Actually no. It would have been better if the 3rd ant said there were two ants in front and two behind him. However, I can still make it work. The trick is in the phrase "one behind the other". Taken literally, this actually only resolves the location of two ants, this says nothing about where the third ant is walking. It's like that silly puzzle where two coins add up to exactly 30 cents, but one of them isn't a quarter, what are they? People struggle with it, until you tell them, sure one of the coins isn't a quarter, the OTHER one is! So one behind the other is just referring to two ants.
So, how can we make it work?
Two ants are walking in a straight line, one behind the other. The leader is walking forward, while the ant behind him is walking backwards. The third ant is walking backwards next to the leader of the previous two ants. The third ant is just slightly ahead of the leader. Since he can see both of the ants with his eyes, he can confidently say that two ants are in front of him. However, because he is really walking beside the leader, within a certain margin of error, he cannot say that the leader is behind him, the leader and him are keeping pace with each other and are even, so there is only one ant "behind" him in a sense that that ant is not ahead.
Oh, by the way:
Labeling the ants 1st, 2nd and 3rd doesn't refer to their location either, just the order in which they spoke. So the 1st ant is actually the one behind everybody and is walking backwards. He has two ants behind him. The second ant is the one I called leader, he has one ant behind him, and one ant right in front of his face - in "front" of him. The third ant is the one I described in the previous paragraph.