Is it possible to have a 11-42 cassette with a 3x9 drivetrain?

Yes it is possible.

It will be a nightmare on your chain and arguably not a good idea.

If you want to use a 9 speed rear derailleur with a max tooth of 36t, you can make it fit a 42t cassette with a derailleur hanger extender such as the Wolftooth Goatlink

Your biggest problem will be the front 3 rings. You will find that if you may have problems when crosschaining. Although, you could argue that you should never be doing that anyways.

Yes, this is possible. Before the current trend of 1x drivetrains took over on MTB, Shimano promoted triple chainsets for their advantages in terms of drivetrain efficiency and choice of gears.

There is some blurb about this here in the context of an 11-speed 3x11-40t drivetrain: https://bike.shimano.com/en-EU/technologies/component/details/dynasys11.html

I would note here that a lot of bicycle drivetrains sold today are not driven by efficiency and smooth cadence, which are some of the merits noted there of Shimano's carefully designed 3 x 11-40 system with 11 gears.

E.g., if we compare this:

11-40t 11-speed cassette, to some of the huge cassettes today:

https://bike.shimano.com/en-EU/product/component/deore-m5100/CS-M5100-11.html

11-13-15-18-21-24-28-33-39-45-51T 11-speed

or

https://bike.shimano.com/en-EU/product/component/deore-m4100/CS-M4100-10.html

11-13-15-18-21-24-28-32-37-46T 10-speed

Then e.g. the Microshift 11-42t 9-speed cassette

https://www.microshift.com/en/product/cs-h093a_11-42t/

11-13-15-18-21-24-28-34-42

or Sunrace make one with the same ratios.

have pretty similar gaps to the current 1x systems sold on bikes costing thousands of dollars, even if they are not as smooth as Shimano's 3x11 kit that was rejected by the market which wanted the simpler but less efficient 1x drivetrain.

If you had a 'touring' type triple such as 48/36/26, it's easy to see how a 42t cog could be useful

http://www.gear-calculator.com/?GR=DERS&KB=26,36,48&RZ=11,13,15,18,21,24,28,34,42&UF=2280&TF=90&SL=2.8&UN=MPH&DV=teeth

The chainline is 1.7° in small/small or big/big. This is much better than the 2.6° you'd get with a single ring setup.

The 8t jump from 34t to 42t is probably bigger than ideal, but it's nothing Shimano haven't exceeded on their 10-speed stuff. I'd probably prefer the 11-40t, or even 12-40t though, which is the same cassette with the top and bottom gears swapped out, so smaller jumps.

The biggest flaw with this stuff is the 15-18 jump, in that you might well spend a long time in the middle gears, and that 20% gap for people who like to maintain constant cadence is a bit of a drawback..... But the same issue appears in some respect in most cassettes. E.g., the 11-36t 9-speed cassette has a 17-20 gap, not to mention the 11-13 gap which is pretty big as well.

So from an efficiency perspective there's nothing really wrong with the idea, in that larger chainrings tend to mean slightly greater efficiency and durability and a 3x9 setup of this kind is likely to be better from a technical efficiency perspective than most 1x setups that people are very happy with. It probably does give you more total range than you need, but that's not a big problem exactly.

As far as the derailleur goes modern Shimano Shadow derailleurs tend for 9-speed to be rated for 11-36t, while the 10-speed stuff and higher goes to 11-42t or more. These are based on what Shimano sells, so this doesn't mean Shimano's 9-speed RD is any different in its capacity from the 10 or 11-speed, except that Shimano sell two different 10-speed RDs for 11-36 and for 11-42t, so it's likely that the 11-36t is not particularly at its most efficient when shifting on 11-42t. (I'd note that the 10-speed 11-42t is probably all but identical to the 11-speed 11-46t model, but Shimano don't sell an 11-46t 10-speed cassette.)

For this reason, while 11-42t is likely to work with a Shimano 9-speed RD rated nominally for 11-42t, I'd be happpier with 11-40t, as it's closer to spec.

Note that you can buy products such as these

https://www.wolftoothcomponents.com/collections/components/products/roadlink

https://www.wolftoothcomponents.com/products/goatlink

to extend derailleur capacity, but all things being equal it might be better to avoid this.

Note also that rear derailleurs have what's called a 'total capacity'. This relates to the number of teeth it can take between big/big and small/small. IME you're quite unlikely to use these ratios on a triple chainset, because you'll tend to get down into at least middle ring, whereas it's more common to find yourself in these ratios on a double chainset. However the figures are there, and given a 11-42t cassette, you've used up 31t of capacity, whereas the 'total capacity' for, say, rd-t4000 is 45t, which would imply just a 14t gap in the front. This implies the angles that the RD will be forced to run across if you use big/big are likely to be bigger than it's designed for.

So like a lot of things yes, it definitely would work, but it might not be totally advisable in all respects and without considering the possible pitfalls, and by sticking to specs issues are less likely, but if your use case demands 3x9 then there's nothing fundamentally against it.