Peter has already answered nicely, but I have a few details to add.
You need to inspect to make that decision. If the 3 new frames are mostly drawn out, and honey (or syrup) is being stored, you don’t need to feed any more. I would suspect that in CT, you have a good nectar flow right now, so I would be very surprised if they need more food. In fact, they may not have used all that you gave them - 2 gallons is a generous feed.
I use 3 golden rules for adding a new box, whether it is a second brood box, or a harvesting super, the rules are always the same. The existing box has:
Fully drawn comb covering most of every frame, and
- The comb is 80% full of brood, pollen or honey, and
Every frame is well covered with bees
If you use those rules, the hive will have enough bees to defend, heat and use the new space you are giving them. Peter said more or less the same thing, I just have the nit-picky details in my rules.
Depending on how strong the nucleus was, and how good the nectar flow is, I usually inspect one to 2 weeks after installing a nucleus. As Peter says, during the nectar flow season (Spring to early Fall), I inspect weekly to check for space, stores, health and swarming intentions. I don’t inspect if I don’t have a specific question to answer. In order to prevent swarming, you do need to inspect weekly at this time of year, though.
I actually leave my entrances reduced all year. On the Flow hive, I block about half of it. My reason is that Tom Seeley (famous bee researcher) has done experiments which show that this is the size that bees prefer if given a choice of sizes. It allows enough ventilation, but also it is not too much for the guard bees to defend from robbers and pests. In late fall and over winter, you will need a mouse guard too, especially in your climate. Small rodents love warm hives with free food!