Perhaps the biggest mistake a PT can make is thinking that DK is just another heavyweight. He's much, MUCH more than that, as anyone who has played a DK master can attest. What more can you expect for the first character to be considered a Snake counter?
Having said that, all three of them make a great case for starting against DK for much the same reasons as they do for Bowser.
Squirtle has to be extraordinarily careful against DK, because the range on his tilts and specials are essentially the size of Squirtle's whole BODY! And forget about shield-grabbing DK. Unless you catch him off of a bad up B landing, it's just not gonna happen; and you'll end up getting grabbed yourself (or worse). However, Squirtle has Kong completely dominated in terms of air speed, and his insta-hydroplane makes a very helpful spacing tool against him as well. DK also has to measure his aggression against Squirtle, because if he pushes too hard, he'll take as much as 70% or more damage off of the up tilt lock. In short, play fast and loose, don't give him a target, and let him make the first mistake.
Like Squirtle, Ivysaur has to keep a watchful eye against DK. Ivysaur's main weakness is early gimps, and gimping is one of DK's specialties. As a major offset for this, however, Ivysaur is one of the few characters that's realistically capable of spacing Kong. Razor Leafs and back airs are essential for the opening phase of the match. DO NOT APPROACH DONKEY KONG. Make him come to you. And once he does, you can start peppering in the damaging attacks that Ivysaur is known best for. Bear in mind that DK does have a "safe" zone - a position in which his tilts outreach your damaging moves and outspeed your keepaway moves. Be VERY careful of this zone, and you'll handle the match very well. Ivysaur, when used properly here, can completely control things.
Filling his role as Godzilla to DK's King Kong, Charizard can also make a viable choice in this matchup. Charizard needs to fight a little differently than normal against DK, because trying to space with Flamethrower/Rock Smash simply isn't going to cut it, here. You're just asking for a back air wall, which WILL win against you. What Charizard's got going for him is his inside game, his grapples (specifically for the setups) and his up-close specials combined with his ground speed. Because DK does NOT auto-sweet spot the ledge, Charizard is also a major threat to the Kong when he gets him off stage. Flamethrower, forward air, and down air are ungodly in the fight against Kong when he's trying to recover. Just beware of armor frames.
All three of them should pay EXTREMELY close attention to his holds, tilts, aerial walls, and off-stage game. This match is a test of patience and stage positioning.