I enjoy designing long distance links so I hope it is OK to expand on Shawn?¡é?€??s exemplary post.
As mentioned LoS (Line of Sight) is critical in this situation if you are using 802.11a/b/g. The link I published below has several calculations that will be of interest to you. Particularly the SOM and Fresnel Clearance Zone, as for the SOM it is what is also called the Link Budget and you typically would like to see a figure in at least be 28dB in order to have a quality link and reserves for bad weather and other unknown interferences. Knowing this as well as your link distance you can determine what power settings, antenna gain, and coaxial cable losses you can manage to get the appropriate SOM (Link Budget). There are many other variables, but this link is a good initial indicator.
The next calculation on that link is the Fresnel Clearance Zone that Shawn mentioned as well. This is a very important concern and should be addressed, especially on long distance links. One thing to remember is that this clearance zone is not just with reference to the ground level but relates to the anything that would interfere with the zone. It could be a building located to the left of the link or a tree exactly in the middle. So from your figure of 3km you would need an obstruction free zone of almost 8m. The one good thing is that this clearance zone assumes the shape of a parabolic figure and the actual 8m is only critical at the median point of the link.
Another concern is existing RF and especially in band radios and interference that are along the path. You should do a site survey to determine if any such RF exists as it can be a show stopper.
I also agree with Shawn as to using 802.11a if possible as it has a better chance against interference as well as having a smaller Fresnel Clearance Zone, which may or may not be to your advantage. You will have to use significantly more antenna gain as 11a uses a higher frequency and that reduces the coverage distance.
802.11b/g if there are no obstructions or interferences should achieve this link without much problem at all. One last band available to you is 900MHz, this is the preferred frequency band if you have limited LoS or obstructions that you need to penetrate.
To get a little more involved with the equipment design is the next step I consider. I try to use an antenna/radio combination that is located at the determined height and position. Having the device at the antenna has many advantages. When considering microwave frequencies you want to use the least amount of coaxial cable and coaxial cable connectors that is possible. Those two items will create a great deal of loss if you are not careful. So having the radio at the antenna helps in that regard. It also allows you to use PoE (Power over Ethernet), which is nice as all you have to run up to the antenna/device is an outdoor Ethernet cable. With the antenna/device being out of doors and typically a significant height above other objects in the immediate area, antenna and device grounding and lightning protection are mandatory as well.
As for actual equipment that will depend on your finances and how important this link is. Reliability does not come cheap. If this is a personal network you may consider something like the Rootenna setup. This link talks about some examples of this. You also can buy the antenna itself if you already have radios/APs that will act as a bridge client.
I only published this link as to point out as to what is available, it is not a recommendation of these products. I also am concerned about the vendor neutral atmosphere of this forum and do not want to violate that.