I doubt you would have any trouble in-game with this set if that is your intention. Before items though, you need to revise at least one moveset issue. Cradily's Infestation cannot be learned prior to Gen 6. It also looks like you have some moves that can only be learned via breeding, so I would double check the egg moves and make sure all of them are compatible with Gen 3.
Now for items. Gen 3 has few items compared to later generations which makes deciding on them a bit easier. Just to clarify, the suggestions below are for your current movesets (I promise I won't add any moveset suggestions unless asked).
Milotic- Mystic Water
Milotic has 2 water moves for stab, an ice type coverage move, and recover. Boosting the stab water is best for sweeping, which Milotic is fit to do with its stats and competitive ability.
Gardevoir- Twisted Spoon
Psychic stab. Calm mind is op. Bug is notoriously weak in Gen 3 so Gardevoir with calm mind to counter dark is pretty much the end of the world for any opponent. Might as well just take that stab since it's your only psychic.
Salamance- Dragon Fang/Lum
Basically you can take this in two directions. Dragon Fang give outrage a better chance to OHKO, and Lum stops confusion. It's extremely important that the OHKO happens, otherwise opponent can switch and you wind up confused. If you intend to give Salamance a lead role and set him/her up, then you can use Lum because TW can be just as important to cover as confusion when setting up and using outrage. However your Salamance knows Dragon Dance which does not raise Sp.Atk (keep in mind that Gen3 physical and special moves are completely different from Gen 4 and up). For this reason, I would definitely run Dragon Fang. Remember that Dragon Dance works against you if you become confused as well.
Slaking- Leftovers/Soft Sand
Your only ground type move (EQ) is on Slaking. It's relatively unexpected so I don't think the opponent would switch (meaning you aren't penalized by Truant). Leftovers are there in case you're planning to take Slaking out after you know the fighting types, ghost types, and focus punch are gone. Gosh Gengar is your Slaking's worst nightmare XD.
I would choose leftovers as well. Good choice.
Sceptile- Scope Lens/King's Rock
This Pokemon has a great moveset and stats for Gen 3 held items. Obviously you're using leaf blade. Leaf Blade has increased crit chance. With scope lens leaf blade's crit chance increases from 1/8 to 1/4. King's Rock adds a 10% flinch chance to every move that does not already have a flinch chance. This is a good way to use Sceptile's speed. I've used both and they are both very effective.
I hope this helped you plan the held items. Be sure to check on those movesets, and you might want to revisit the Gen 3 move types before doing so. Good luck and remember: Gen 3 is best!
Here is a link to the Gen 3 move types: http://pokemondb.net/pokebase/40995/what-types-in-gen-iii-are-physical-and-which-are-special