I cheated
The man takes the chicken across first, leaving the fox and grain together on the other side.
He returns and gets the fox, but when he deposits the fox on the other side, he takes the chicken BACK across, so that the fox and chicken aren't left alone together.
He drops the chicken off back on the other side, picks up the grain, and takes it across to deposit with the fox.
Finally, he returns to retrieve the chicken and takes it to the other side.
At no time were the fox and chicken left alone together, nor were the chicken and grain.
At no time was more than one of them in the boat with the man simultaneously.