I wouldn't recommend outside panels unless you like putting on your galoshes to reset a branch breaker. The weather is also not nice to costly AFCI and GFCI breakers. There's no problem getting 400A meter-mains (featuring dual 200A main breakers for two 200A subpanels) or even Ranch Panels, which do the same but also provide an 8-space mini panel hung off one of the 200A breakerss. Great for generator interlocks, solar, or subpanel breakers to outbuildings.
For the service wire, 150’ isn't quite far enough to have to worry about voltage drop. Service and feeder to a dwelling is entitled to a favorable derate at 83% of service size. So 400A x 0.83 = 332A. You need wire rated for at least 332A at 75C thermal.
That works out to 600 kcmil aluminum. (340A).
For your 200A feeders from meter to panels, 250 kcmil aluminum. Some people think 4/0 but they are misapplying the 83% rule (it doesn't apply to every single feeder).
They said it can be direct buried (in fact, they kinda recommended that over running wire in conduit).
That helps with wire theft. Other than that, it's a disaster.
They recommended parallel 4/0 AL triplex. Will this work: https://www.platt.com/p/2013103/triplexed-aluminum-conductor-600v/4a22a1urd1350x1000?
Never listen to a power company about wiring advice. They follow a completely different rulebook called NESC which applies to their side of the service splice point.
But I would not even think about paralleling without talking to your AHJ (permit issuer/inspector). Under NEC rules, paralleling requires equipment at the supply end that is rated for paralleling. I doubt the power company splice will be. The practical problem is that if a wire breaks, how would you know? All the current is on the remaining wire, overloading it.
Now granted, a typical 400A residential service is unlikely to see more than 200A for long enough to overheat a wire. If they were doing an overhead service drop, they'd probably use a single 4/0 - they get to do that under their rules.
I'd run two of these. And I assume I'd then pull these through a conduit
Dual 4/0 is the correct size for paralleling a 400A service (since the 83% rule applies here).
However if you run them through a conduit together, or run them right next to each other, you degrade their ability to cool. As such, the wire loses capacity, and is derated to 80% of the highest ampacity allowed at its highest thermal rating (90C). Thus 4/0 drops to 164A and 250 kcmil is 184A. Since you're paralleling you need 166A x 2 per the 83% rule. 2 amps shy... I would ask your AHJ.
Or you could lay them a bit apart and use 2 conduits. One hot-hot-neutral pair per conduit, for 300.20 reasons.
I would not attempt to cram dual triplex into 3” conduit. You won't enjoy the experience.