For addition of ordinates questions, is it ok to sketch the two initial functions on the axes in addition to the resulting sum graph?
Also will we ever be asked to add 3+ graphs together?
I would personally recommend that you sketch the two initial functions first. It allows you to easily see all the significant points, such as intersections and x-intercepts. If you want to leave all 3 functions on the same axes, be sure to label each function (what I used to do is use dotted lines to draw the initial functions, and then draw the final function as a solid line). Otherwise, you can always carefully erase the two initial functions afterwards
Addition of ordinates questions have been relatively uncommon on VCAA exams in the past (they have been known to appear on SACs though), so I would say that it is unlikely that a question asking you to sketch the sum of 3 or more functions, but even if such a question is asked, the best method would be to do them one by one (i.e. adding two functions at a time).