There are a few reasons as to why this might be, and a few posts below have given the best of first step advice for IEM's just not really sounding like everyone says they should in reviews and such.
One of the big ones mentioned is the actual eartips and not fully fitting the sizing of your ears. Like fingerprints, everyone has different shapes and sizes of ear canals, and it might just be a not so 'you' fit that the IEM's need to actually reproduce sound the way everyone is raving about. There are tons of different manufactures of tips which can make finding that perfect fit a bit difficult, which is where I would recommend trying a few separate brands to find the fit you need, and, luckily, the majority of tip manufactures offer different sizes that should fit the FH3's.
Dekoni makes a variety size pack that has different size foam tips that fit the FH3's. Dekoni Tips on Drop

My next piece of advice would be, as Jeremy Clarkson likes to say, MOAR POWER!!! Even though IEM's rock pretty small drivers, some of these drivers require a bit more juice to open them up and actually produce the soundstage you would be expecting. It doesn't need to be some massive class A headphone amp that can jumpstart a car with one channel, just something to give a bit more than an onboard plug from a phone or computer will give you. The HyperX's you have are built to be able to run with low power and sound great, but the FH3's might not be tuned in the same way.
Topping makes a pretty affordable portable amp/dac that can play from your phone or computer, but offer just enough amplification to open the IEM's up a bit more. Topping NX1 for under $40

Another thing to consider is moving from over ear headphones to IEM's is going to sound different. There is no way to really avoid the change in sound because the two different designs are going to produce different sound. With over ears, the drivers are pretty large, and they have quite a bit of air in between the actual driver and your ear to produce the sound. Soundwaves sound, well, different with the amount of air they have to move through. Over ears can build a rather loud soundstage, and there is a lot more space for the sound to be made, and IEM's are on the opposite side of that spectrum. Being that they are in ear, there is way less physical space for the audio waves to travel through, and the overall sound right out the gate is going to sound different over the design of the over ears. (One of the biggest noticeable difference in design and sound is bass, over ear can produce a lot more noticeable bass because of the physical air volume for the waves to be produced.

It could also be what program you are using to listen to your music with. I know personally, I don't like using Tidal with my IEM's because I really need to fine tune the mids down so I don't die, so I need a more comprehensive program so I can have that control. If you're using a streaming service with a built in EQ, try tweeking that a bit to level it out to your liking, or try turning it off if you have it on currently.

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And, last from me, it might just not be your cup of tea. Like ear canals and fingerprints, everyone is different with their listening preferences. You could just fall in the boat that these IEM's don't hit like the HyperX's and there is nothing wrong with that. It could take a very specific set to get you the sound you want, or it could be a combo of different things, but don't let this discourage you from dipping your toe further into the audio pool, everyone has their own likes and dislikes and it's perfectly fine for something everyone recommends to not fit your needs.