Is sinus bradycardia always abnormal?

No. Sinus bradycardia may just mean that your heart rate is slightly below the calculated and established "average hr" for the population. Now, if your hr is so low that it is causing symptoms, is not able to maintain adequate blood pressure and / or does not rise with increasing activity, then it is abnormal and you need to be evaluated.
No. The heart rate will slow in well conditioned athletes due to increased stroke volume from the heart. Long distance runners can have heart rates in the high 30's and low 40's and be totally normal. Also some people on beta blocker medicine will have low heart rates.
Not necessarily. There are superbly conditioned athletes whose resting heart rates are inordinately low, but these are a rather select group of people. For most others, a good cardiologist will find out what is wrong.