I have a 3" strut spacer with 285/70/17 BFG TA at 45 psi

I'm getting really bad clunking when going over speed bumps.

Is this fixable or just an inherent problem with 3" spacers?

 

might want to check all of your torque specs. i am starting to get a slight clunk on my 2.5. tightened the top nuts a little and took some of the clunk away so i am gonna have to disassemble and retighten the nuts on the spacer

 

It could be the Upper Control Arm hitting the coil spring on full droop, problematic with 09+ with more than 2" spacer lifts. Nothing can be done with that unless you replace the UCA with a Camburg Uniball, however you then can't use any rims with more than 4.75" backspacing (regardless of rim diameter) which negates any factory rims. But with that shallow of a backspace you may start to encounter significant tire rubbing issues with the fenders at full lock, depending on tire diameter.

 

You should post a few more pictures of your truck, Im considering 285's as well and I would like to see it.