I argue that this is a terrible idea. If I go into full out on just 3 abilities, I'd like some real buffs and not some compensation that had been laid to Jayce before he got any small buffs.
I know that not being able to max all of them is aggravating but if you must do 3, then do the 3 and have the last 3 upgrades for his unused one so that its there.
The thought sounds about right but its just a nerf and as someone who spent a lot of time with Udyr when I first got onto League, I know that its difficult for him because he already cannot get up to his max potential. Udyr has no real ultimate and only has his 4 abilities (Jayce having 6 and every 1 upgrade helps 2 abilities). The only way I'd want to fix Udyr is to make all of his abilities go up to 4 max and save the last 2 for a upgrade-able passive. Imagine if Udyr started out with a passive that allowed +5% and could grow to +7% and +9% after two upgrades.
His Q should be able to deal max damage at a level 5 standard faster as well as being able to level up all his abilities with no real drawbacks as they will just level up faster.
If you don't like it, I'll understand. :/