Why do some ADSL wireless routers have 3 antennas?

If you have access to the manual it may explain why there are three.

Most of the three antenna routers are using the 802.11n spec. The goal is better range and less impact from barriers like walls as I understand it.

Here’s a little bit about it on the wiki

And a quick discussion with more available via membership.

Different manufacturers use different designs – it’s that simple. Netgear uses two antennas, while the more recent Cisco and Linksys routers hide the antenna in the case and have no visible antennas to break.

Technically, they all work about the same; it’s more of a design issue than it is a functional requirement.