Just replaced my BB UN 53 after 3k miles and 10 months. Its great to be able to do it yourself and all but I think in this day and age to only get 3k commuting miles out of a square taper BB is ridiculous. I know that they are relatively cheap to replace but this level of mileage is way too low in reality.
BB should last you way longer than that, and all mine do (UN53s, 5500s, 6500s). Don't have an answer, could have gotten a bad one. The sealed cartridges really don't need any maintenance, might have just gotten a bum one.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
That's VERY early failure in my experience. I've had several Un 52, UN72 and UN73 bbs last over 30,000 miles on my and friends bikes and still be working well when replaced as part of an upgrade.
One possibility is that your bb shell is poorly faced and may be putting a bending moment on the bb when it's torqued down. As a test, see if the spindle is noticably harder to turn when the bb has been installed than when it's is free of the frame. There will probably be some increase in friction but if the difference is dramatic, that may be the cause.