On Sun, 5 Aug 2007 23:17:47 -0500, "DanG" wrote:

These things probably have traps in the bottom. It would seem
that you are losing the water trap on the last 3.

They can dry out from non use. Simply add water.
They can leak out through a crack or a hole or be eaten up by
acid/chemicals. Install new traps.
They can be sucked dry because of a venting error. Replumb.

Definitely a p-trap is not holding water, which acts as a barrier to
the odor of the sewer backing up.

But the problem could be lack of a vent to the roof on the sewer line
right before the traps. Or if there is one, it is stopped up. That
causes siphoning which drains your p traps.

The city also might help you by checking for backups on thier side.