Secondary fermenting 3 gallons in a 5 gallon carboy.

My lovely wife found a beer brewing kit on our local buy nothing group and, knowing that I was researching getting into brewing mead, snagged it as an early father's day gift.

The kit comes with pretty much everything I would need for mead but the bucket I would use for primary and the carboy for secondary are both five gallons. I'm not totally sure if I'm ready to go for a batch that large to start but I was thinking that 3 gallons might be more realistic in terms of cost.

From cursory research it seems like a five gallon bucket would be great for primary fermenting 3 gallons but I can't tell if the five gallon carboy is going to have way too much space for secondary. I don't want to buy smaller carboys if I don't have to but I also don't want to mess up my first batch if I don't have to.