Encounter Design - Combining Monsters, Traps, etc.

[ClavierCavalier]

I'm trying to figure out how to combine traps and monsters in one encounter. For example, the ettercap uses traps, as do kobolds. Does one just treat the trap as a creature of the same CR?

Another idea I had is to have combat on a bridge/ledge by a body of water. If anything gets knocked in, a hidden aquatic creature, maybe a swarm, attacks the unfortunate creature. How does one figure the difficulty of such an encounter?

How about a good three way fight? Every creature on the other two sides are opponents, so does one use the combined CRs? They will technically get help from the creatures, too.

[blahpers]

1. Treating the trap as a creature of the same CR makes the most sense.
2. The creature in the water isn't a threat until the PC interacts with it (or vice versa). I wouldn't count it as part of the encounter unless it becomes relevant.
3. Getting help from other creatures doesn't usually change the CR of an encounter short of GM or designer fiat. Award the party for any enemies that represent a challenge to be overcome.