Hi MARS
You are correct in saying that there is just a single energy register for all three phases' energies put together (Note that there are two different sets of active energy registers though- AENERGY/RENERGY and LENERGY).
To continuously monitor the energies in all three phases, it is necessary to monitor the total active energy (sum of all phases). But if you can afford to monitor individual phases' energies one by one, then ADE7754 has necessary configuration options to help you do that.
In the table snippet attached below, you can see that by setting one or more of the LWATSEL bits, you can include one or more phases into the active energy computation. The WATMOD options help with using different formulae for use in different 3-phase configurations.
With respect to RMS results, there are individual current and voltage RMS registers for all phases (Address: 29h to 2Eh). So, that shouldn't be a problem for you.
I just wanted to point out that a lot of our recent ICs have independent energy registers for all three phases, along with a lot of other feature and performance improvements. So, if you still haven't narrowed in on an IC for your application, it will be worth considering some polyphase ADE ICs such as ADE7758, ADE7878A, ADE7880, etc..
Regards
hmani