This issue has popped up a few times since BG3 came out due to the way that game has allowed people to scribe higher level spells into their spellbook, but the idea is to be as restrictive as other classes.

As a DM, I allow players to 'scribe' spells that are higher than their level into their spellbook earlier, but I do not allow them to be prepared until you have sufficient wizard levels. So you can drop that third level spell in your book - but until you're a 5th level wizard you can't cast it. This is mostly just a 'convenience' rule as it allows the wizards to plan out their spellbook acquisitions better in one place.