The Lexicon Audio interface has 2 mono inputs, so with 3 interfaces you would get only 6 inputs (it can be enough if you need to record only a few instruments separately, but you won't still be able to record individually all the drumbrute sounds).
Same for the Native Instruments Komplete Audio 6 USB Interface, it features only 4 analog input (the last input is SPDIF, which is stereo but digital, so it's useless for this task).
Anyway, I would advice you against the aggregate device solution (handled natively on Mac, or with Asio4All on Windows), it's a possible source of problems (syncronization between multiple devices). I would rather spend the money on an interface with more inputs (maybe with ADAT, if you plan to expand your hardware setup).