Oh X3. How much I have to say about you.
Well, let me say that X3, while having a ton of bad design choices, manages to still be a solid, fun game. It isn't quite as good as X2 and not even close to X1.
Now then, level design. There are alot of empty areas in X3. Gravity Beetle for example, has a massive amount of flat, boring design with absolutely no enemies to stand in X's way in some areas. The overuse of elevators in X3 is another bad thing. What is the point of these? They are only breaking the pacing of the game. Thankfully, you can skip them, but if you don't have the dash upgrade, you'll have a much tougher time doing so.
Enemy AI. Most of the enemies pose no threat at all. The enemies that munch on dirt in Tiger's stage, the enemies that take a second or two to realize X is an enemy before realizing they should fire at him, and those absurdly pointless green barrier enemies. They could have done alot more to the enemy design, like speeding up some of the enemy behavior and removing the pointless enemies, or at least making them a threat.
Boss AI. This is absolutely a joke. First of all, the hitboxes for the bosses are mostly just huge squares. Some cases, taking damage from a boss will leave you wondering what exactly it was that hit you. Second, I would like to point out something. How many enemies do you think ram forward in order to damage X? Let's have a look here.
Hornet: When he tries to sting X.
Buffalo: When he isn't shooting ice at you, he's ramming. I'm sure everyone knows this guy can be exploited to constantly ram into the wall by jumping everytime he turns towards X.
Beetle: He starts glowing and rams forward.
Catfish: In his despiration phase, when he absorbs lightning, after a few seconds he will ram towards X.
Crawfish: In his despiration phase, he will turn around and very quickly ram towards X.
Rhyno: Is constantly ramming. Also, the miniboss in this stage constantly rams.
Tiger: In his despiration phase, he starts glowing and rushes towards X before leaping into the air.
Bit: Jumps into the air before ramming with his saber.
Byte: Shoots a gravity based projectile onto the wall before ramming.
Vile: In his ride armor he will ram towards X before punching. If you wait to fight him later in the game in the Doppler stages, he will constantly ram here as well.
Doppler: Goes into the air and uses this red goopy barrier thing before ramming towards X.
So there you have it. Obviously they could have put more effort into most of these bosses. Though, I guess to be fair, Hornet and Tiger can be overlooked. Hornets rushing to sting something makes sense and Tiger isn't really ramming into X like everyone else.
Weapons: This is a mixed bag. There are some good, useful weapons, and there are other weapons that can even manage to be completely useless.
Parasitic Bomb: Not too bad actually. You can use it on an enemy and just dash through them. Charged, it could be a bit sped up.
Frost Shield: Again, not too bad. It's extremely slow, but it guarentees Life energy dropping from enemies. Can't really say much about the charged ice.
Gravity Well: The most useless weapon in the entire series. You fire it and it absorbs small enemies. The same enemies that die with a single charged shot. It also takes an age before it actually absorbs them and usually you're already past the screen before anything happens. Charged is still a joke. There really isn't a point in using this at all.
Acid Burst: Pretty useful. It's a fairly strong weapon and can be aimed up or down so you can take out flying enemies or enemies waiting below. It also guarentees weapon energy dropping from an enemy. Charged, it's not that useful. It bounces around randomly and is kind of slow.
Triad Thunder: Pretty good. It can be used as a shield and in some crouded areas, it can be used to clear out enemies. You can also rotate it around, allowing it to be used a bit longer, and it can be rotated 3 times. Charged isn't really useful. No real point to charging this one either.
Spinning Blade: Good. It's strong and fires two projectiles, stacking damage with each other. Can be used to clear enemies very quickly. Charged isn't too good. If there are enemies around X, you can rotate it and take them out, but it's never really necessary. Additionally, this weapon can be used to replace Gravity Well as Hornet's weakness, allowing to kill him quicker.
Ray Splasher: Not very good. It's not very strong and not really practical to use in stages because it fires in random directions. It's good for a good deal of bosses though. Charged, it's worse.
Tornado Fang: Not bad. Not very practical in a stage and is pretty weak, but you can use it to slow some enemy animations, allowing you to quickly bypass them. Charged isn't really any different.
Music: Megagrunge X3. The instruments are awful. While the music isn't too bad overall, it always has boring, grunge guitars backing most of the songs. This OST would have sounded better on the Genesis for example, but even still, alot of the tracks could use more polish, especially the Password theme, Neon Tiger and Tunnel Rhyno's stage.
Well, that's about all I had to say. Sorry for the wall of text. I do hope people would be interested enough to read that.