How is a 120 degree triple more balanced than a 90-degree four? Maybe smoother than a flat-crank big-bang 180-degree I4s, like supersport motorcycles tend to have... (although they are going to 90-degree crankshafts, and offset crank locations compared to the cylinder centerlines, anymore)
In a motorcycle arena, I3s do tend to be torqier for the same displacement class, due to having larger individual pistons, and fewer cylinders allowing larger cylinder pitch while still having a shorter-length cylinder block at the same time.
The first motorcycle I ever fell in love with was a triple... a T595 Triumph Daytona, back when Daytonas were full-size bikes, not 600-class, as Daytona is now.
But they have to have a 60 degree firing order with a long back-side lag, or a 120 degree firing order, and they don't have the aspect of an opposed bank of cylinders to cancel their inertia like an H6 does, which does allow for an even 60 degree firing order by staggering the power strokes with the intake stroke of the piston opposite, in a boxer arrangement where both pistons fall toward each other, and rise away from each other.
An I3 does not have that opposition, and to balance the engine to be smooth, it probably requires a balance shaft, where an H4 or H6 has inherent natural balance.
An I3 has to be vertical. Subaru doesn't do vertical inline engines much. If it were to be a "flying brick" I3, like a BMW K75 motorcycle, it would have to be offset to one side of the car, or the other... and either in Japan/Britain/Australia, or in US/Europe, the engine would be on the same side of the car as the driver, and un-balance the car, laterally.
An H4 has a 4-cylinder/4-stroke firing order, and is one cylinder pitch shorter in length than an I3, and is much lower than a vertical inline anything of similar displacement and power. It also is more balanced than an I4, by the effects of the horizontally opposed pistons. An H6 is even more balanced than any 4 cylinder, and smoother than an I3, by allowing 2 banks of I3s to be firing out of phase with each other, with one power pulse every 60 degrees of crank rotation, rather than 120 degrees.
Most automotive 3-cylinder engines are purely economy engines, not performance engines like Triumph triples, which I happen to like, BTW... and they make sense transversely to cut down on width, compared to a 4. (not that I don't also like BMW's new 1600CC transverse I6, which is barely wider than the 1300cc I4. I6 has the same 60 degree firing order as an H6, and both act very nearly to half of a V12, BTW.)
BTW, Justy's 3-cylinder was vertical, and transverse, completely different than Subaru's H4-AWD layout, more like a Geo Metro FWD car, which also had a 3-cylinder engine. Transverse vertical engines save space by going to 3-cylinders. It is a different layout with Subaru's longitudinal layout.
Think of it this way... An old gold-wing flat 4 is shorter in length from the front of the crank to the flywheel, and shorter in height as well, with a lower center of gravity than a longitudinal Triumph Rocket 3 engine. Even if both were similar power, or even if the I3 were 25% less displacement, compared to the flat 4... the crankshaft would still have to be longer, and the head would have to be taller than the boxer's top intake manifold.
If we were talking about a transverse layout, I would almost suggest dropping the Rocket 3 engine into a lightweight car, and having fun with that... with the gearbox slung along side the crankshaft, inside the crankcase, not having to be bolted to the back of the engine. A tiny little RWD mid-engined car, like an ariel atom style thing, would be interesting with a rocket 3, adapted to drive a transverse differential, instead of a longitudinal driveshaft.
I would love to build a Morgan-style 3-wheeler, with a Triumph Rocket 3 motor, which would be epic fun.
But as for what you are suggesting, I don't see how Subaru can bring a whole new body platform, even if pared down from established Impreza hard points, to an even smaller size, AND produce a unique engine (an I3), with AWD hardware, and sell it for 15 thousand dollars new. The R&D for all of that would cost too much to pay back with a $15K market price point. Sharing the engine with already established engine platforms would cut costs, and might still have trouble selling with AWD on board for $15K, but maybe $17-19K.
Most high end motorcycles are more than 15K new anymore, with much less material and fewer component parts involved.
I too like uniqueness, and motorcycles. I like boxer, brick, and I6 BMWs, and 955-1050 triple Triumphs, and other V-twin bikes. I am not a huge fan of transverse I4s just as a matter of course, like YamaHondaKawaZuki, and now BMW S-RR seem to do to copy each other every two years. Interesting tech, but the same thing over and over.
Don't get me wrong, I don't dislike I3 engines... I am just not sure why Subaru would put one in an economy car, when they aren't paring down from an I4, they already have a superior H4 engine layout.