If you only bought it this morning it's too early to panic yet. PayPal payments normally go through raight away, but perhaps there's a time lag when using pay in three.
To ask the obvious, I take it that you paid through eBay's checkout, and not as a guest, and didn't pay the seller direct outside of eBay? Also that you meet the criteria for eligibility set out in the pay in three FAQ.
By the end of the day I would expect PP's system to provide more complete information on the transaction. If it doesn't, phone their cusromer support.