The TZT3 side scan currently uses both of its sounder channels in order to run sidescan.
It would need a third channel to run downscan, OR, a better software update that took advantage of a nice multiplexer in order to run a 3 in 1.
I'd be interested in seeing how the garmin and simrad 3 in 1 's are setup. My guess is it has a nice multiplexer setup like i imagine, but who knows.