How to catch Froakie
I'm dividing Froakie into his own section because so far he seems to be the rarest of the new Kalos group. Most of the players working on the Kalos Celebration Timed Search are constantly stuck at the stage that asks you to catch a Froakie just because they can't find them. And unfortunately, that's because it's actually quite difficult to find. But with a little help, you can increase your odds.
Pokémon GO has changed the way characters appear in the world a lot, but there are still biomes that create higher odds for certain types. Froakie is an Aquatic Pokemon, so it will appear more often in aquatic biomes which are typically found near rivers, lakes, oceans, or other large bodies of water. If you have one near you, you might want to plan a trip there (safe and socially remote). If they have any PokéStops out there that you can lure, even better. And as an aquatic Pokémon, Froakie will be more common in rainy weather.
Using incense will really help you the most no matter where you are but especially if you are near water. But it still takes a lot of luck, so you'll want to increase those odds as much as you can. Eggs may be the key here as all entrees spawn in 5 km eggs more often. Try to put them in incubators and hatch them as quickly as possible in case they contain Froakie or other Kalos starters. Then spin more stops to get more for the duration of the event (until 22 p.m. on Tuesday, December 8).
How to catch Fennekin and Chespin
And all of the above is true if you're looking for the equally elusive (but not nearly as much) Fennekin and Chespin. Fennekin is a type of fire so he will spawn more often in “desert” biomes. These aren't very common, so you probably can't research them. However, a clear, sunny day will increase the chances of Fennekins spawning, and this is quite common.
Chespin is probably the easiest to catch of the whole group, spawning in weed biomes found all over the world. And with the types of grass boosted by clear, sunny weather, they can also appear next to that Fennekin you've struggled so hard to catch. And just like with Froakie, Fennekin and Chespin are more common in 5k eggs for the next day, so try to hatch them or at least in your egg inventory before you finish.
And these are all the tips and tricks for catching Froakie, Fennekin, and Chespin in Pokémon GO.