In the top, right corner of the Campaigns page, you'll notice the Compare button. If you're trying to measure the effectiveness of different campaign types, or other differences between mailings like subject titles, use Compare to view the data. This is a great tool to use for any A/B testing that you want to do.
The default display is in Scaled View. This plots all stats scaled to 100%. So you can compare the types of stats equally between all three campaigns, regardless of whether they were sent to the same amount of people.
There are two other compare stats views available, that you can use to slice the data in different ways.
You can click on the % button at the top to view the stats displayed in Percent View. This view compares the percentages of all stats to each other. So a campaign with a 100% engagement rate would exceed other compared mailings, even if it was sent to the smallest number of contacts.
The No. button will display the Actual Number View. Here we're comparing the actual number of contacts that viewed, engaged, bounced, etc. So a campaign sent to the most people will overtake the display as the largest bar on the graph.
You can click through all of the tabs of the different metrics, to compare any stat type between the campaigns: accepted, bounced, viewed, engaged, shared, growth, forwards, and marked as spam.