It says mana cost is reduced to 3
But it should read by 3.
The mana cost oc the first decorated card is reduced by 3.
I was wondering why they wanted my topdeck Omniscience to cost only 3…reducing by three seems a lot more fair.
It did seem a little bit broken 
After playing the card, reducing TO 3 seems more appropriate since it reduces the cost only of the first card drawn. That means, when you cast it, you have to wait the following turn and even then, the first card drawn must be a decorated card. Any decorated cards drawn afterward by spells or effects will not have their cost reduced. For example, you fill a deck with decorated cards + Overwhelming Splendor to combo with topdeck and fill those 3-cost cards with 3 mana, essentially filling them up. Well this is a nonbo because the cards drawn by OS will not have their cost reduced.
An effect that can trigger at most once a turn is not broken at all.
I think @Oktagon_Support should look into this and fix the card as it will make it interesting at least, but still not OP.
Thank you for the feedback, this issue will be investigated!