Hm, it seems to be that if you select an expression and hit Group it groups with the expression directly above (and ends up just below this expression in the group).
If you want expressions 1, 2, 3 in this order in the group, first ungroup them all and then pick expression 2, click group, then click expression 3 and group.
First of all, ungroup all the expressions added. Then add the expressions in the order you want them to be grouped.
After this is done,go to the last expression and then select Group.This would group your last two expressions and hence repeat the same step hitting the Group button .
So, this would group all your expressions in the order you want them to be.
Hope this helps.