It's nothing new, there have been many threads about it over the years.

If they don't have availability for all three at the same price, they will price them all at the higher price.

It's also totally possible after one ticket was sold, the system reallocated the inventory and opened up another lower priced ticket, so you could do them one at a time and not three at once.

They know most people won't go the one at a time, and will gladly pay the extra for three.

It's quite common, and anybody booking multiple tickets needs to check the price of just one to see if they are the same, if multiple tickets cost more, it may pay to book them all one at a time.