Before anyone tells me the automotive website Paultan.org has already done the comparison review between these 3 cars - Yes I am well aware of that. It just so happens I had the opportunity to try out all three cars back-to-back so I decided to write a comparison review of my own.
Also present during the test drives were 
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Design
Let me first concede that every person has their own poison in terms of design so this is just my personal observation.
For the exterior, I feel the Myvi has the best proportions and reasonably sporty, something that appeals to the younger demographic of car buyers which comprises a large portion of Perodua's customer base. A close second is the Picanto, for which I feel Kia has done an outstanding job making a small car look aggressive without falling into ricerhood, and let's not forget that pair of beautiful headlamps. I hated the Iriz's design when it first came out, but it has kinda grown on me. It's kind of a mixed bag to be honest, if nothing else, it is actually quite unique and I can see how it would have it's fair share of fans.
On to the interior, the top spot goes to Picanto, no contest. It is neither outrageous nor super modern, but it tries to be none of those. The design is up-to-date and logical in it's execution, with a restrained sprinkle of flair here and there. The Proton and Perodua tries, and largely they are effective. But Picanto is just that much better, at least to me.
Practicality
Myvi has the biggest cabin and the biggest trunk space, so it is the champion in this category. To be sure, Picanto is one full segment smaller than both cars so it's not a fair comparison. As for the Iriz, the amount of cabin space or the lack thereof is comparable to my Fiesta or a Mazda 2. Is it a huge deal? not really, but the most inexcusable part is the laughably small trunk space that is even smaller than the Picanto's.
However, Myvi front seats are the most uncomfortable, followed by the Picanto because of it's small seats. The Iriz's front seats are the best among the three, comfortable with plenty of side bolsters to hold you in place during high G lateral movements ie: hard cornering. I doubt anyone would be comfortable sitting for long periods of time in the rear seats of any of these cars.
NVH (Noise Vibration Harshness)
Despite it's petite size, the Picanto actually feels the most grown up in terms of NVH. The cabin is adequately insulated and the ride is stiff but not harsh. The engine noise might be a bit of strain on the ears when worked hard, but it is nothing worse than the other two cars. The Myvi has the worst noise insulation among all three, with rear passengers repeatedly reporting hearing water splashing sound coming from the rear wheel well when driven on wet roads.
The Iriz, which used to has the worst NVH, has improved quite alot over the years especially with modifications carried over from the new Persona, a sedan based on the Iriz platform.
Safety
All three cars come with 6 airbags and stability control on their highest trims (Picanto has only 1 trim at the time this post is written), but Iriz is edge out slightly by seatbelt reminders on all 5 seats on the Picanto and Myvi. It's not a huge problem, but it is what it is.
Also out of the three, only the Picanto has rear disc brakes and objectively the best brakes among them.
Drivetrain
I cannot stop gushing about how good the engine and gearbox combo is on the Picanto. the gear ratios are well-mated to the engine and the throttle response is quick especially at low speeds.
Myvi is a Myvi, with the typical Perodua ho-hum approach to "let's make this car as economical to drive as it is to build". Flooring the throttle produces a harsh engine note but without the acceleration to match. Fuel efficiency prioritized over fun to drive.
My experience with the Iriz is mixed. The CVT response seems to be improved from my test drive years ago when it first launch but it is still behind in responsiveness compared to CVTs of other makes such as Honda. This is compounded by the infamous Campro throttle lag that is still making itself noticed, and the underwhelming power to weight ratio of the car.
This anecdote during our test drives sums it up - when I was driving the Iriz down a straight stretch,
Handling
Despite the spotty quality issues that often plague their cars, Proton is known to make excellent handling FWD cars. Iriz is no different - from the body control, steering sharpness and levels of grip. What surprised me more, once again, is how near the Picanto comes. In fact I'd argue the Picanto has the sharper off-center steering, which might be good or bad depending on your usage. Good, because it is less vague and easier to make mid corner adjustments. Bad, because it makes the car a bit more twitchy at high speeds. Both has similiar ride quality, stiff but not trashy - more European than Japanese.
The Myvi's ride and handling is a major leap from it's predecessor. While it does not corner as well as the other two cars, I feel that it has the best balance between comfort and responsiveness. In fact Perodua has done so well this time around with it's chassis and suspension I think it is better balanced than the other Japanese makes.
VERDICT
When we completed the test drives and got together to collect our thoughts, we unanimously agree that the Picanto presents a very strong case and shocking value for money. It is simply better in every aspect except for the cabin space due to it being the only A-segment car compared to the other two which are B-segment cars.
The Myvi is a fantastic all rounder. It is not particularly outstanding in any aspect, but it does many thing well enough that it is easy to see why it is Malaysia's favorite car.
The biggest problem with the Iriz is the drivetrain, particularly the CVT. It is just not very well thought out and you have to work hard to extract some measure of power out of it, and not in a good type of way like VTEC. The Campro VVT is a matured engine, Proton simply has to do better with their tuning. But if you're not the fast and furious type, the 1.3l variant is still a solid car with plenty to offer, and a perfectly fine and safe daily commuting car. But if your daily route involves driving on the highway, go for the 1.6l variant.