[Project_Mars]

To start off, Heatran and Tentacruel completely shut down CAP3's typing. Heatran's combination of typing and stab render it immune to both CAP3's types (versions that don't carry Earth Power have no way to threaten it back though), and Tentacruel is resistant to both typing due to being part poison, and can fire back with a SE water attack. Jellicent and Gastrodon can also wall it real well due to resisting poison with their secondary types, and Gastrodon has Ground STAB to really waylay CAP3. The typing also gives Dugtrio an easy revenge kill, again due to the Ground STAB, and it will be a guarentee thanks to Arena Trap. Tyranitar deserves mention as it enables sandstorm for ground types, resists both CAP3's STABs, and can fire back with a SE Rock STAB. Finally: Any good Ground-Type can pretty much be a nice check, due to hitting for 4x damage. (Landorus comes to mind) Bulky Waters are also good at blocking CAP3, but if it does wind up getting a poisoning attack, must be wary of that. (If CAP3 has a higher attack than Sp. Attack, then Gyrados would love to come in on CAP3)

When it comes to what could be threatened by this, Volcarana does take neutral damage, but has both STABs walled. Grass and Steel types in general must be wary of the fire-type part of CAP3, especially Ferrothorn, Forretress, and Scizor, who have their other typing (grass and bug) double resisted by CAP3. In addition, Lucario has his fighting type resisted, and would thus have trouble setting up in CAP3's face. Out of the Grass and steel types only 3 carry a typing with a SE STAB on CAP3: Celebi, Metagross, and Jirachi, so CAP3 does have to pay attention to those, especially Metagross, who tends to carry Earthquake.

So in closing:
Counters right off the bat: Heatran (Total Immunity + Potential incoming Ground attack), Tentacruel (is a wall that Resists Both Types + SE Water STAB + Part-Poison), Jellicent (is a wall that Resists both types + SE Water STAB), Gastrodon (is a wall that Resists both types + SE Ground STAB), Tyranitar (Resists both types + SE Rock STAB. Has Perma-Sand) Dugtrio (Resists poison + SE Ground STAB. Arena Trap means if you have CAP3 out when dugtrio hits, you are very likely to lose it based on typing.)

Potential Counters: Starmie (Resists Fire and can fire back with two STAB attacks.) Gyrados (Resists Fire, has SE Water STAB, and Intimidate.), Politoad (Resists Fire, has SE Water STAB and perma-water. CAP3's least favorite weather.) Ground Types in General (resist Poison and is the 4x SE STAB vs CAP3).

Will it succeed despite threats/Is this list of threats acceptable? That's the whole point of the challenge, neh? I think it will, depending on the role we take this with. Again, Heatran can only threaten with Earth Power, so if it doesn't carry that both then its STABs are resisted by CAP3. The Water types listed as hard counters are fine, Tyranitar is fine, and Dugtrio as a revenge killer is fine, since CAP3 resists three of the more common priority moves (bullet punch, mach punch, and ice shard) The potential counters are all generally good 'mons, so nothing really out of the scope of the project.

Pokemon Threatened by CAP3 based off typing alone: Ferrothorn (Both STABs are resisted, with a double resist on Grass, and is 4x weak to fire), Forretress (Both STABs are resisted, with a double resist on Bug, and is 4x weak to fire), Scizor (Both STABs are resisted, with a double resist on Bug, and is 4x weak to fire), Steel-types and Grass-Types in general, with exception for Heatran (Fire resists steel, and Fire/Poison both resist grass. Celebi, Jirachi, and Metagross are less vulnerable, especially Metagross.)

Are these targets "unavoidable" to threaten based on type alone?: The 3 named ones are for sure, yes.

Are these Pokemon targets we want CAP3 to hit?: Oh heck yes. Being able to consistantly scare away the three mentioned types is an amazing thing, and CAP3 will shut down Scizor from sweeping based solely on type (Resists Bullet Punch, X-Scissor, Superpower, Brick Break and U-Turn). The ability to do so is a huge plus for CAP3, which is a great reason already to pick it.

Where to go with CAP3 from here now that we know what it's counters/threats are?: An interesting question. It would be interesting to see a Fire-type Tank, and the part-poison typing could help with that. A Support role could work too, again due to the resists. If you want to go attacker, it doesn't entirely need a ton of bulk to be real survivable, as outside Ground, Rock, and Water, it isn't damaged too terribly by much, so it could do okay as a sweeper. Really, picking a direction for CAP3 is a bit harder because there's several roles it could fill fairly efficently.

also: first real wall of text?