The simplest solution would be to set width: parent.width/4
for each of the checkboxes. If you want to keep the checkbox width set at some known value, you could instead set spacing: (parent.width - 4 * checkboxwidth)/3
on the Row. Note that this will cause the elements to overlap when the parent is narrow.
If you're targeting Qt 5.1 or higher, you may want a RowLayout. I'm still on 5.0, though, so I can't help you there.
Yet another way to do this would to be to put each CheckBox in an Item. Each Item would have width: parent.width/4
, and each CheckBox would have anchors.centerIn: parent
. This would give a half-width margin on the far left and far right, which may or may not be desired.