We want to create robot that will localize itself by the signals of wifi routers. Which sensors should we buy to detect strength of 3 WiFi signal? Which of following is necessary for us? http://www.dfrobot.com/index.php?route=product/category&path=45_80 or can be any other more suitable variants? We are using arduino as a platform.
Any WiFi system should be able to report the signal strength (in dBm), but the question will be how quickly you need it to report values and how accurate a measurement you are looking for.
The main problem with what you're suggesting is that the relationship between signal strength and distance is not necessarily linear. Antennas produce an interference pattern which can make the strength change depending on your orientation to them.
The second problem is that the signal strength can be affected by the environment: humidity, nearby metal, buildings, and even your body can affect the strength of the signal (if not directly, then through interference).
The only way to really do it is by heavily characterizing the wifi signals in the area you want to operate in and hoping that it doesn't change too much over time.