This is how we can use 3 toggle buttons with 3 icons

toggle_with_three_icons.dart

ToggleButtons(
  children: <Widget>[
    Icon(Icons.ac_unit),
    Icon(Icons.call),
    Icon(Icons.cake),
  ],
  onPressed: (int index) {
    setState(() {
      isSelected[index] = !isSelected[index];
    });
  },
  isSelected: isSelected,
),


ToggleButtons(
  children: <Widget>[
    Icon(Icons.ac_unit),
    Icon(Icons.call),
    Icon(Icons.cake),
  ],
  onPressed: (int index) {
    setState(() {
      for (int buttonIndex = 0; buttonIndex < isSelected.length; buttonIndex++) {
        if (buttonIndex == index) {
          isSelected[buttonIndex] = true;
        } else {
          isSelected[buttonIndex] = false;
        }
      }
    });
  },
  isSelected: isSelected,
),


ToggleButtons(
  children: <Widget>[
    Icon(Icons.ac_unit),
    Icon(Icons.call),
    Icon(Icons.cake),
  ],
  onPressed: (int index) {
    setState(() {
      for (int buttonIndex = 0; buttonIndex < isSelected.length; buttonIndex++) {
        if (buttonIndex == index) {
          isSelected[buttonIndex] = !isSelected[buttonIndex];
        } else {
          isSelected[buttonIndex] = false;
        }
      }
    });
  },
  isSelected: isSelected,
),


ToggleButtons(
  children: <Widget>[
    Icon(Icons.ac_unit),
    Icon(Icons.call),
    Icon(Icons.cake),
  ],
  onPressed: (int index) {
    int count = 0;
    isSelected.forEach((bool val) {
      if (val) count++;
    });

    if (isSelected[index] && count < 2)
      return;

    setState(() {
      isSelected[index] = !isSelected[index];
    });
  },
  isSelected: isSelected,
),