Vlada,
There are a fair number of articles on using the Random Sample flow step to accomplish this. You can start here:
http://www.marketingrockstarguides.com/how-to-run-ab-email-tests-in-marketo-505/
Essentially what you want to do is have that Group 1 already split into a random set of static lists or even just a flow step:
Random Sample of Group 1: 20% ...
Smart List for Flow: if member of list, Groups 1a,b,c, 2, 3.
Send Email to Static List if Group 1a, 1b, 1c...Send Emails X, Y, Z
Send Email to if Group 2, Send Email Winner X
Send Email to if Group 3, Send Email Winner X