I came up with this placement.
The idea was to maximize the time a zombie spend in the trap area.
In order to do so, i force the zombie to walk in a U path around most of the trap.
To further expand the time they spend, one can also use an electric fence wire
through the trap area. The arrow slit also allows for repairs from above.

Red = Path zombies will take through the trap.
Purple and yellow = Can be removed, and additional blade
traps can be placed where the purple block are.
I usually set the trigger plate to 1 minute.
Caution: I've had red console errors popping up when trying to
send too many zombies through the triple trap.
A single trap should be good enough for daily use, but i'd not trust
it for horde nights.
edit: I also placed this trap upside down. (advanced rotation). This
seem to work better when enemies have lost their legs.
The idea was to maximize the time a zombie spend in the trap area.
In order to do so, i force the zombie to walk in a U path around most of the trap.
To further expand the time they spend, one can also use an electric fence wire
through the trap area. The arrow slit also allows for repairs from above.

Red = Path zombies will take through the trap.
Purple and yellow = Can be removed, and additional blade
traps can be placed where the purple block are.
I usually set the trigger plate to 1 minute.
Caution: I've had red console errors popping up when trying to
send too many zombies through the triple trap.
A single trap should be good enough for daily use, but i'd not trust
it for horde nights.
edit: I also placed this trap upside down. (advanced rotation). This
seem to work better when enemies have lost their legs.