The three antennas interfere constructively to increase the range. Unless you want to get 3 identical antennas? If the router is lying flat, the best way to position the antennas is to have all 3 pointing up. If in a vertical position, have one point to the left, one to the right, and one straight up. You can also use a piece of aluminum foil to create a parabolic antenna which helps a lot if your router is in a closet or against a wall. Routers emit radially.
Also, a few questions:
1) do you have any wireless B devices?
2) What type of encryption are you using?
3) Are there a lot of networks around you?
A few suggestions:
1) Switch to WPA2 AES only.
2) Adjust the channel width to 40 MHz (if possible), or auto 20/40.
The DIR 655 used to have impressive range but due a policy called the good neighbor policy implemented, they severely reduced the range to reduce interference in the 2.4GHz range. That's why 5 GHz routers are starting to become quite popular. Luckily, I have a revision A router which lets me use the 40 MHz mode.
3) If you have no b devices, switch it to g and n mode only.
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