explain omega-3 fatty acid increase 'good' HDL , without increasing the bad LDL'

It sounds to me like you're interested in knowing the mechanism of action.

According to one study published long ago, long-chain omega-3 fatty acids from fish oils or omega-6 fatty acids reduced LDL synthesis, and increased fractional rate of catabolism of LDL, or a combination of both.

In normal human subjects, virtually all the apoprotein B in LDL is derived from the intravascular catabolism of VLDL; however the proportion of VLDL apoprotein B that is converted to LDL varies widely. This precursor-product relationship implies that in normal humans reductions in the rate of synthesis of LDL can be mediated either by a lower rate of VLDL production or by a reduction in the proportion of VLDL apo B converted to LDL. The observed reductions in LDL apo B synthesis by dietary omega-3 fatty acids could, therefore, result from a primary inhibition of VLDL synthesis, an enhanced direct clearance of VLDL remnants, or a combination of both these mechanisms.

The exact mechanism by which omega-3 fatty acids lower triglycerides is still unknown, but they reduce the hepatic synthesis of VLDLs, which are almost entirely triglycerides, and accelerate lipoprotein lipase, which hydrolyzes triglycerides.