Verified answer

The gameplay is pretty much the same. Here is the Pokemon that aren't in ruby, but are in Saphire:

Lotad

Lombre

Ludicolo

Sableye

Seviper

Lunatone

Kyogre

Here is the Pokemon that aren't in Saphire, but are in Ruby:

Seedot

Nuzleaf

Shiftry

Mawile

Zangoose

Solrock

Groudon

And now Emeralds differences from Ruby and Saphire:

Different Pokemon sprites.

Team Aqua/Magma are more heavily involved in the game.

There is also inclusion of many new areas including Magma Hideout, Mirage Tower, Desert Underpass, Marine Cave, Terra Cave, Studio Cave & finally and most importantly, the Battle Frontier.

The Battle Frontier contains 7 buildings which all incorporate a different style of battling where you have to battle through multiple trainers to get to their leader's, The Frontier Brains. These battle styles include Rental Pokémon, Random Rooms, Relying on your Pokémon's Nature to battle, Tournements and many more...definitely a way to challenge any trainer

The Gym Leaders & Elite Four have also been updated with different Pokémon and a whole new gym leader! To make things even better you can rebattle the Gym Leaders up to 4 times via the Match Call feature

There are many more things to Emerald including Johto Starters, Contests, Breeding & Ability Additions, Rare Legendary Pokémon, Pokémon from other Regions and many many more.

Pokemon included in Ruby/Saphire, but not in Emerald:

Surskuit

Masquerain

Meditite

Medicham

Roselia

Lunatone

Zangoose

All of this information could have been found by looking through Bulbapedia and Serebii.net. Next time do a bit of digging before coming to YA.

Pokemon Ruby Vs Sapphire

Difference Between Ruby And Sapphire

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RE:

Key differences between Pokemon Ruby, Sapphire and Emerald?

I've only played Ruby, so I don't know how different they are. Is Sapphire basically like Ruby, but with Kyogre instead of Groudon (and Team Aqua instead of Magma), or are there lots of different Pokemon and stuff?

And is Emerald just a combination of the two (which means it's not...

Yes, what you said is right. There are also version-specific pokemon for each one.

Sapphire - Kyrogre and Team Aqua

Ruby - Groudon and Team Magma

For Emerald, you can capture both Kyrogre and Groudon as well as the main legendary, Rayquaza. There is also Battle Frontier.

Check out the first source to find the version-specific pokemon. The second source has the Emerald changes from Ruby and Sapphire.

Like you said, Ruby is Groudon, Sapphire is Kyogre, but Emerald is Raquazza. The pokemon will still be pretty much the same but a few differences. Do get Sapphire though, I think you need both Groudon and Kyogre to get Rayquazza (possibly not).

I have both Ruby and Sapphire the story is very similar but Pokemon are different, I think you should get emerald and see the difrence also because you can trade the point of Pokemon is to catch em all