using three different IP's for ASAv C4.large instance?

Hello all, just a question that is probably easy for you guys to answer but not so clear to me. I am running an ASAv, we are currently in a testing phase and at the moment I am using the ASAv C4.large instance from the amazon market place, Now at the moment I am using two ENI's that are assigned to two different subnets (using this as an inside and an outside address) I can see that the ASA v can have up the 3 ENI's on the c4.large image however when I add the additional interface it just pulls an IP from the inside interface subnet (which is the default subnet it was created in) Is this and expected function or am I doing something wrong, I thought I could assign this 3rd ENI (the management eNI) to it's own subnet? It let me do this with the outside interface when the instance was created? Am I missing something?