Evil Magnetic Sorcerer

[X2FNbadX]

This card cannot be normal summoned or set. This card can only be summoned by tributing 3 DARK monsters on your side of the field. The cards used to summon this card are attached to this card as if this card is an Xyz. This card's ATK and DEF are the sum of the levels of the attached monsters x 300. Once per turn, you can detach and destroy 1 monster attached to this card to attach another monster you control to this card.

[Sander]

So, what the hell is the point of this card? Other than it being a bad beater that will never be Summoned because nobody with a half a brain will Tribute 3 DARK monsters for this. ESPECIALLY when Destiny HERO - Plasma exists.

 

Personally, I'd suggest that you'd remove the Xyz effect, because it's just plain redundant and is not actually realistic, and change it so that the card gains ATK and DEF equal to the combined levels of the tributed monsters. Then modify the last effect somehow.

 

Anyways, with that out of the way, your OCG also needs help. This is the correct way to word this kind of effect:

 

Cannot be Normal Summoned/Set. Must be Special Summoned (from your hand) by Tribtuing 3 DARK monsters you control, and cannot be Special Summoned by other ways. This card's ATK and DEF becomes the combined original Levels of the Tributed monsters x 300. *insert other effects here*

[Trebuchet MS]

Because OP designed this card around being able to modify its stats by changing what was "attached" to it, I would suggest this alternative:

 

Cannot be Normal Summoned/Set. Must be Special Summoned (from your hand) by banishing 3 DARK monsters you control, and cannot be Special Summoned by other ways. This card's ATK and DEF becomes the combined original Levels of the monsters banished for this card's Summon or by this card's effect x 300. Once per turn: You can return 1 monster banished for this card's Summon or by this card's effect to the Graveyard; banish 1 monster you control.

So instead of making this a pseudo-Xyz monster, the cards are now being managed in the banished zone from where they can be juggled about while still keeping the Graveyard outside of the necessary stat calculations.