Swap the bulbs side-to-side. If the too-bright situation follows to the other side, the bulb is way off-spec and should be replaced.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.

It is very difficult to come up with a reason for a bulb shining too brightly.

Could that be a fog/rain light?

I would take all the bulbs in question and put them directly across battery voltage to see how bright each one is under ideal conditions.

Perhaps the other two are too dim.

If the bulb in question is too bright, I would get rid of it - too much brightness could mean too much heat and an expensive repair down the road.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '12 XC70, the first three are mine, heh, heh, 525,000 miles put on 10 bricks

http://cleanflametrap.com/tony/tail_light_color_code_v0.htm

This might help
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html http://cleanflametrap.com/tony/

I am no electrical expert, but corrosion somewhere is the most likely cause. Have you looked over the circuit board, wiring, contacts and connectors? Clean everything including the ground. If the circuit board is really bad, replace it.

240 sedan tail lights are notorious for problems. My 90 DL sedan tail lights gave me fits at times!