At the top of the IMM5710E form (Page 1, under the heading) it states that, "if you need more space for any section, print out an additional page containing the appropriate section, complete and submit it with your application."
Therefore, print a blank page of page 3 of the form and fill your two other employment experiences. This means you will use the number 2 & 3 spaces to avoid confusion. Number 1 is for current. The rest are for previous.
Scan the page, save it as pdf and add attach it under optional documents section in your application account in MyCIC. You might perhaps want to write a brief letter to accompany it, explaining the reason for the additional page.
Your Reason: I needed more space under the employment section for my previous activity/occupation experiences.
To make it easier, download a new IMM5710E form, fill in page 3. Print it out, scan, save as pdf, and attach.
To print page 3. Go to File>Print>Choose Pages>3-3>Print
Cheers!