Since the panels have an integral diode, the three-way
cigarette-lightr adapter setup you describe should work
just fine.
There are two problems with overcharging a wet lead-acid battery:
The extra power goes into electrolyzing the water into gas (which
goes away) and into heating the battery (which can cause thermal
damage if the heat gets too great - and losing water also loses
cooling fluid that would otherwise help distribute the heat.)
But with only 3 * 0.17A = 0.51A in full sunlight, if you've got
a car-battery sized battery you definitely don't have to sweat
thermal runaway and molten lead or boiling water. B-)
And with, say, 5 solar hours per day you only get about 2.5
amphours per day, so you'll be doing well to stay ahead of the
leakage. If you check the water ever few months you don't have
to sweat it going dry.
So I wouldn't worry about a regulator. Just check the water
from time to time.
If you have an 85 amphour deep-cycle battery and discharge it
down to 20 amphours (about 74% discharged) it will take it a month
or more of sunny days for the panels to bring it back to full
charge. So undercharge, or sulfation from chronic undercharge,
might be more of an issue.