Same SSID, same security, just put your other wireless access points on a different IP address of course. And put them on opposite channels.
Say router is 192.168.1.1, have it broadcasting on channel 1
Put your 2nd access point at 192.168.1.2, have it broadcast on channel 6
put your 3rd access point (if needed) on 192.168.1.3, have it broadcast on channel 11
You won't get true roaming...as you take a laptop and walk across the house...it will hold onto its prior association as long as possible..and then suddenly flip over and latch onto the access point near the other end of the house you just walked to.
People don't usually walk around much though...basically you take your laptop, go sit somewhere..and fire it up..it'll latch onto whichever is strongest.
The other thing you're probably thinking of...."WDS"...that's where they basically act as repeaters. Since you're playing with DD or Tomato firmware, you can get that feature. You end up chopping your bandwidth down considerably though...dunno if that's an issue.