Hi Susan,
kamik86 summarized it pretty well. Airlines generally price seats based on anticipated capacity for a given flight. If a flight is more likely to be full on travel day, the prices are higher (Christmas vacation, spring break, etc).
If a flight may not be full, then the airline begins to discount the seats at the price point they think will entice people to buy. As such, only a few seats may be at the lower price, with the idea that as seats are bought, the remaining capacity will be in more demand. As such, your flight likely had some seats at the $134 price range, which left one or two remaining at the lower price.
While it might seem intuitive to give you the remaining seat at the lower price and the others at the higher, this may or may not be happening. Some airline booking systems will average the price of all three tickets and spread the calculated per seat cost evenly, then display it (like a $100 and two $200 seats average to $166.67 per seat, so that would be the per unit price). On the other hand, some systems may just price all three seats at the next higher bucket, since the convenience for most travelers is enough to not make separate reservations. Which version WN's (Southwest Airlines) systems use, I don't know.
To expand on this a bit, guessing the capacity requirements is tricky, which is one reason the low-cost airlines tend to release their schedule 6 months prior to travel day, while the legacy airlines release theirs 330 days to travel. The pricing on the latter is generally higher until the six month mark, where more reasonable statistics can be found for anticipated capacity.
Plus, this is why airline prices fluctuate so much. The system guesses final capacity based on sold seats versus previous year's flight metrics to either raise or lower prices. If a lot of seats are sold earlier, then the price goes up and stays there until closer to travel day, when they may dip if there is no further activity for a while. On the other hand if no one gets a ticket for the flight early in the cycle, then the airline may post the flight at a sale rate to generate buzz.
There are lots of factors involved in airline pricing, but this is why you see higher prices for three seats versus one, and why ticket pricing fluctuates.
Good luck!