Hi Erica,
There are a couple of ways to accomplish this:
1) You can follow Milind's and Keith's advice and do three different emails (however, you do not need three different programs, just three campaigns) and set up three different campaigns. First, divide your list (if you want it random, just send one email to the people with names A-H, then H-P, then P-Z or however. Set up your campaigns using "member of list" for the SmartList and then send email in the Flow step to run as a batch campaign.
2) Pull the whole list into Marketo, use one campaign and three emails - Again, use the "member of list" filter and just select your one list. Then, in the Flow, put the "send email" action and use 3 choices. The first will be "If: Random Sample is 33" then "send email T1. The second if is, again, "If: Random Sample is 33" send email T3. This
Help Article explains it step by step and with screen shots.
Either method will work fine, it just really depends on which you prefer. I tend to go with the second method because I think it gives a better view of the emails stats so that I can compare trends more easily in the "email" tab in Marketing Activities.
Hope this helps!!