That is the dilemma you face when forced to replace a tire. In that situation I usually go to a tire dealer that has lots of used tires and pick one out the same size, and as much tread as the remaining tires on the car.
I thought I was in the same situation as you the other day. For a week I could hear something clunking when driving slow or on very smooth pavement even at highway speeds. I began to think wheel bearing, but I finally felt around all the tires and found a self tapping bolt in one of the tires. It was near the edge and I'm sure if I went to a Toyota dealer or a Costco they would not fix it. About 6 months ago I ran into the same issue on my daughter's car. Too close to the edge, we can't fix it. At that time I decided enough of this and bought a $6 tire plugging kit
. I put a plug in her tire (an expensive Michelin), and it has been fine since. The other day, I did the same with the tire on the Camry. I pulled out the bolt with a pair of vice grips, cleaned the hole, with the special tool in the kit, and installed a plug. It is holding just fine.
So next time that is what I suggest doing. For the situation you are in now, I would just leave it as it is, and replace all four when the three wear out. It is kind of a waste of that newer tire, but what else can you do. Buying a second new tire just digs the hole deeper, unless you want to start replacing the tires in pairs instead of as a complete set. I don't like doing that. I guess your other option if the tires are nearing wear out is to replace the remaining three now...