Think about it this way, if you are running one of these other 2 major competitors, you know it may work this way.
One company can lower it's prices, which will trigger the others to do the same. If each company lowers it's prices, then there will be no change in costumer preference. Basically, if each company lowers it's prices 25 cents for a 2 liter, then the same proportion of costumers will be buying from each company as they did before the price drop. The only change in the numbers then, will be the change in price which results in a loss of profit.
Anyone who works at these companies who has as much sense as you or me will know that if they drop their prices, the other 2 companies will also drop their prices within a couple of weeks. Therefore, anyone in charge of making these decisions will look forward a month or two, and understand that if they drop their prices, they will just trigger a price drop from the other companies, and the result will be a net loss of profit.
These companies are able to think into the future too, they know that while lowering their prices may make them the best deal in town for a week, once the other prices are dropped, they just lost the only advantage they got from dropping their prices.
So, the companies know that lowering their prices will only trigger a race to the bottom and ultimately a loss of profits, so none of these companies is going to want to be the first to pull the trigger.
If another company drops their prices first, and if (this is a big if) that price drop actually hurts the profits of my company, then I will follow up with a price drop, but this is how everyone is thinking, they will all wait for everyone else to do it first.
When you have a large market full of diverse companies, it's more likely that someone will try to undercut someone else, but when you are working in a market with only 3 major players, it's very easy for this "uncommunicated, non-planned, price-fixing" to take place.
Except that the consumption tax will be tacked onto the very same product. I have doubts whether prices would drop at all for the reasons I listed above, but even if they did drop, adding 9% consumption tax will offset any change in price.You're completely ignoring the fact that if the US went to a consumption tax, ALL OTHER TAXES on new products are gone, except for State Taxes. That alone will drive down their cost and make their products cheaper.