So, for the most part I agree with what Rising_Dusk laid out, but there are still a few things I would like to mention. As has been said many times already, Heatran is one of the more obvious counters, and I think this is a very good thing. The same goes for almost all Rock and Ground types. Tyranitar is probably one of the most important counters we can give it, and I think that it is very important that it not have a way to easily get around this. In fact, sand as a whole should give it significant problems. While I would not be 100% against letting it beat a few ground Pokemon, it should not have the ability to scare them off consistently.
Next, I think that CAP3 being a stop to itself would be great. Now obviously for this to happen, it needs to wall itself but not be able to hit back hard with any coverage move. If it is worried enough about losing to a fellow CAP 3 that it chooses to run HP Ground or something, then it would really show how much of an impact the typing we chose could really have.
The big group of Pokemon to think about though are Water Types. I will also agree that CAP3 should have a way around water types, however, water types are such a huge group of Pokemon that saying it should beat them as a whole is kind of unrealistic. No matter how we do it, ground/water Pokemon, most notably Gastrodon, should always be able to beat it. Additionally, I feel that any offensively inclined Water type should at least be on level ground against it. The most important group of Pokemon for CAP3 to threaten, in my opinion, would be bulky waters. Jellicent and Tentacreul are key examples of bulky waters that look like counters but I think we should be able to beat. I think this goes hand in hand with what reachzero said about having it beat Rain. No one Pokemon should really be able to counter the entire strategy, but being able to take out the bulky backbone of these teams would give it a cool niche, even in a hindering weather.
This however still leaves one of the most important Pokemon up in the air: Politoed. However, I think the way to deal with this is fairly similar to what I said above. Offensive Politoed and any Politoed that specifically prepare for it should be able to take down CAP3. However, Bulky Toed should not be able to win the battle with just Scald.
I think that having the ability to beat some of these water types, but not all, would give is a great niche in the metagame, without being too ridiculous.
<jas61292> I record everything
<nyttyn> prove it.
<nyttyn> WHAT WAS THE VERY FIRST THING I EVER SAID IN THIS CHANNEL.
<jas61292> [06/02/12 | 12:43] <NyttyN> Huh what deo you know shelll smash did get banned
<nyttyn> well congratulations jas your stalker nexus has reached an all time high.