what is your reasonable price? For what it is worth, my Rei Traverse 35 accommodates a 3 liter bladder quite easily and carries very comfortably. There are several others, as well, generally costing $100+.
I have the Gregory Zulu 30 and it's the best day pack I've ever owned. It will support a 3L bladder.
One note, I know you said "reasonably priced" and that's a different amount for each person but one thing I would strongly suggest - 3L of water weighs just over 6½ pounds. If you plan on carrying a full bladder, don't go too cheap on the pack. Day packs are smaller to begin with and the lower-end packs just don't distribute weight as well as higher-end packs. This is definitely a time when I would say to buy the best you can afford.
Otherwise you'll find yourself hating that reasonably priced pack because it just hurts your shoulders too much.
Thanks for reaching out!
Generally speaking, a daypack that is 25 liters in volume or above will hold a 2.5 liter water bladder. You've gotten some good advice in this thread already but we wanted to offer a couple of options that will fit your hydration bladder.
In the 25-30 liter range:
In the 30-40 liter range:
Hopefully this helps, thanks!