3rd edition also simplified and expanded statistics, getting rid of the old 1-25 system and bonuses that only kicked in at 15 or higher in most cases. Now, for example, every 2 points of Strength above 10 gives you +1 on melee weapon hit rolls and +1 damage. It worked like this for all stats, give you +1 for every 2 points you put into the stat above 10. Stats also have no upper limit, with things like Great Wyrm Dragons having stat scores in the 30's to 50's.
You also got a increased stat point every 4 levels.
In addition they added in a feat system, a Prestige Class system, and more. Every 3 levels, or at certain levels for some classes, you'd get a feat. Think perk in Fallout, or other similar games. A special thing you could take, based on class levels/stats/skills/etc that allowed you to improve something. Prestige Classes replaced kits, only you didn't have to choose them at the start for the character. Each one had special unlock requirements, some mechanics based, like ability to cast seventh level spells plus X ranks in Knowledge: Arcana, and others character history/story-based, like "Must have singlehandedly destroyed an undead creature of equal or higher level."
3rd edition was a fantastic gaming system that saw a 3.5 update to its mechanics and was in use for quite a few years before Wizards of the Coast moved on to 4th and now 5th Editions. 3rd Edition lives on in the Pathfinder system, which expanded upon the open source material and is widely regarded as D&D 3.75 Edition.