Hey, welcome to the forum! Good idea, getting the K-3!
With macro, here is the thing. Pretty much all macro primes these days have great IQ. Sigma 70mm, Sigma 105mm, Tamron 90mm, Pentax DA 35mm f2.8 ltd, Pentax DFA 50mm, Pentax DFA 100mm. Even older manual macro primes are quite good (The Pentax FA and F 100mm have the same optical layout as the latest DFA 100mm). All of the ones I listed already go to 1:1 magnification at minimum focus distance, which is usually more than enough for simple macro work. Only beware budget macro zoom lenses (which often don't actually go to macro magnification). Some older macro lenses only go to 1:2 and need extension tubes to go to 1:1.
Here is what to watch out for.
a) Focal length: DA 35mm f2.8 limited has 1:1 magnification, as does the DFA 100mm. But the difference is that the working distance (distance between lens and in focus subject) at that magnification will be very different. The DA 35mm ltd has to be very close to the subject to achieve 1:1, it comes so close you are almost touching the subject! So you can imagine it is not the best choice if you want to photograph snakes, skittish things, or even if you have a lightsetup (camera and lens can cause shadows, or reflect in the subject). This is why the DA 35mm ltd, even though it is an amazing lens, if only recommended to advanced macro photographers, who know what they are getting into (the rendering between 35mm macro and 100mm macro will be different, especially due to perspective, so there is a reason why 35mm macro exists). But the lens can still be used for everyday non-macro work, so its not a bad investment. Focal length around 100mm is much more comfortable and is usually recommended to macro beginners.
b) Features: All modern macro lenses are sharp, have a flat field of focus, etc. But not all macro lenses have the same features. Only the latest Pentax DFA 100mm is WR - which can be important in some cases, but is pointless in other cases. Then there is aperture ting - the latest DFA 100mm does not have one! This can be a problem if you want to use the lens on older film bodies or on macro bellows (these can be used to reach extreme macro magnification), since you cannot select the aperture you want. Then there is focus limiter - some older and third party lenses have this. Its great because it limits the AF, so the camera doesn't search for focus in the macro range, when you are doing non-macro work; or the other way around. Focus clamp is another feature - this just grabs the focus, so it doesn't get accidentally bumped. Can be useful in studio work. So think about which of these features you need. It is too bad that the latest DFA 100mm WR (there is an older DFA 100mm as well, not WR and slightly different construction - but same optics), has wonderful aluminium barrel, WR, QS, but doesn't have focus limiter and aperture ring. But its optics are amazing, and it comes for a very fair price.
c) other equipment. You will probably need a tripod and lights. This is not necessary, but if you want to go truly macro, you need to think about these things eventually. But take your time! You will figure out what you need as you go along.
This forum is a great resource, there are a couple extreme macro and macro at any cost threads out here. They can be very helpful with the setup and settings. Good luck :)
Edit: Oh yeah, about Pentax. Pentax uses the K-mount, also known as KAF, sometimes as KR (K-Ricoh) or ZK (for Zeiss lenses in K mount, but these are now discontinued). So if you buy a lens, make sure it is K-mount, for Pentax, or just a Pentax brand. m42 mount lenses are also popular, but they need a simple glassless adapter. No automation on m42 lenses, though. You can use some other mounts with adapters, but adapters with glass will degrade the image quality a little. And some lenses you cannot adapt at all. Please give us more details about the lens you are interested in. I don't know that specific lens
