Not sure how they compare to SotN, but the three DS Castlevania games all seem to get a lot better in terms of how combat is done and sub-weapons upgraded.
Dawn of Sorrow: Sub Weapons are Souls. One is casted like a spell/ranged attack, another is an effect that is triggered and must be held and will continuously drain mana for an effect and the last is a passive benefit.
Souls are aquired from killing a creature. Sometimes it's soul will fly about the room and your character absorbs it. Now do that, for the most part, 8 more times to get the maximum power of that soul, whatever it's effect is. Combined with the fact that some souls are RIDICULOUSLY low drop chances, combined with mana being slow to regenerate, the game lacks a decent kind of flow for it's sub-weapon system.
Portrait of Ruin: System is tweaked, you control two characters. One being a Jonathan with various additional weapons or attacks as his sub-weapon, the other being Charlotte whose sub-weapons are her spells. These are aquired in more varied ways than the previous game, you can find certain ones in levels, buy certain ones in shops, as reward for quests.
Charlotte's spells are static and only scale in damage with your stats, but Jonathan's weapons must be mastered. Hitting an enemy with a sub-weapon and killing it will improve the mastery by 1. As you raise the mastery, the weapon does more damage and will grow in size and effect. It's a far better system than the chaotic randomness of Dawn of Sorrow, and certain sub-weapons for Jonathan are low on the mana side, which helps as mana is still an arse mechanic in the game as it still regenerates too slow.
Order of Ecclesia: Previous system of main attack and sub-weapons that use mana combined. The game now has two attack buttons and whatever abilty you've aquired can be mapped to it. Duel-wielding swords? Go for it. Sword and a Fireball? Sure! As long as you have the mana, you can attack away. Mana also restores itself incredibly quickly when you've not attacked for a second or two, allowing for strategic use of dodging and attacking.
The game calls it's system "Glyphs", glyphs are found in a similar way to the previous games, dropped from monsters at a rare rate, aquired from quests or progressing to story, just around in the world (Sometimes the glyph is making a hazard in a room that you must navigate to find the glyph and absorb it). Mastery is back, but is not limited to each glyph, but instead the element or effect the glyph has, be that Fire or Blunt (For hammers). This can lead to you developing one or two types of attack far above the others and having trouble when said attacks do next to no damage to a particular foe (Such as Holy damage to an Angel).
Overall, the combat system over these three games just evolves in a really great way, each time the developers think about ways to tweak and improve the way the player fights. With the slightly confusing dual-character gameplay of Portrait of Ruin (Having that second character out was more of a hiderance than help), they succeed in this aspect. And considering Order of Ecclesia could be hard as hell, it needed to improve the system too.