Etherchannel (in Juniper terminology AE or LAG) does NOT work active-backup, but always work active-active basically. You could do some things with LACP settings, but far too deep a discussion for your situation.
You need to add the physical interfaces into the AE config. I suggest you look at Day One EX book which will have a section on how to configure an AE/LAG (LACP/802.3ad is Optional) and follow that. Not even sure how your EX3200 config worked?
Be sure to set AE count as well - system parameter.