It depends on the power and its bonuses...I would usually 3 slot something like hasten...I'm no numbers expert but the bigger the number you're trying to enhance, the more the enhancement does...for example 3 slotting Hasten is worth it as its base recharge is high, though 3 slotting say, Twilight Grasp with recharges is probably not as its base recharge is less.
Never played a D3 personally so if nobody else has any advice for you here, you could try boards.cityofheroes.com (the USA boards) and check out the Defender guides section, should be some D3 guides with IO advice floating about somewhere.
Defiant EU
Quaver: Kinetics/Sonic Defender
Semiquaver: Sonic/Kinetics Corruptor
D3 is a great defender build, and you can do lots with it. If you have some money or have come across some nice drops on the way to 50, you can go for some recharge, and get Nightfall and Tenborous Tentacles coming round a bit quicker for some AoE carnage. Taking Hasten for this (and 3-slotting)is also a good idea 
Nice recharge sets can be Red Fortune or Positron's Blast for example. Basically just look at the types of set bonuses you want to go for, and then see how easily achievable stacking a fair amount of them would be.
Hope that helps.
Tux
@Fail (Used to be @Tux) and @Tuxedo Infinitus Defiant/Freedom/Champion
Favourite Toons: Prosper [Ill/Rad] Controller :: Infinitus [Fire/Elec] Blaster :: Pocket Dynamo [Fire/Shield] Scr :: Fast [Cold/Sonic] Def ::
Inspire [Plant/Storm] Controller :: Quality - SS/Fire Brute :: Double Down [DP/Kin] Corr :: Pwnz [Fire/Cold] Corr :: Fail [Fire/Son] Corr
I dinged my first 50 wednesday night with my D3, Dark Requiem, and I'd like some advice on invention sets to make my life a little happier.
Currently I'm using single enhancements at levels 45 and 50 and want to improve a little.
Also is it really worth putting 3 of a kind in the same power? Mid's seems to show only a small % increse when the third is added.
Thanks in advance
Main Heroes: Dark Requiem 50+2, EarthGlade 50, Natures Wrath 50, Imperion 50, Spectral Echo 50, MydKnight 42, Flash-Point 42, Cascade Effect 39, Scirrocco 35, QuantunStarburst 41, EarthLight 35, Silver-thorne 32, StormFrost 35
Up and Coming: , DragonStorm, Metalworks