Congrats on your new NG3! The third allen screw needs to be loosened before making any lateral adjustments with the other two screws, then re-tighted when you finish your adjustment. It just holds the adjustable bridge piece securely in place. This mainly comes in handy when you are changing strings...so the piece doesn't fall out.
If the bass is brand new, I would encourage you to first dial the truss rod in perfectly to your taste and your climate before messing with the saddles. The saddles are probably already pretty good from the factory. Of course, nothing wrong with you dialing them in exactly as you want them if they are too high or too low for your playing style. But the truss rod needs to be set correctly first, or you will be chasing your tail.
The most valuable lesson I learned in doing setups is:
1. Tune the bass to playing pitch.
2. Adjust the truss rod, so that the relief (i.e., amount of bow in the neck) is correct for the string tension. In general, you should be able to just barely slide a business card or a thin guitar pick under the lowest string at the 7th fret without deflecting the string while you (and/or a capo or a second person) are fretting the string at the 1st and 13th frets, holding the bass in playing position (not lying on a bench). If you are buzzing anywhere below the 5th fret, the neck is too flat and the truss rod must be loosened a little. If you are buzzing up above the 12th fret, that has nothing to do with the truss rod. Keep reading.
3. After every truss rod adjustment, no matter how small, start at Step 1 again. (Don't forget this important step!)
4. Now that the truss rod is set correctly, adjust each saddle so that you just barely start buzzing when playing at your normal playing strength above the 12th fret, then raise the saddle just a hair at a time (a half turn or less) to make the buzzing stop when you are playing as aggressively as you foresee yourself playing. Of course, individual tolerance for buzzing may vary, so adjust to you personal tolerance level. Note that if there are two saddle height screws, they should be adjusted in unison by the same amount so that the saddle remains level, parallel with the body of the bass and bridge.
5. If you find yourself buzzing below the 5th fret, check the neck relief again and adjust the truss rod as needed. A humidity change may have caused the neck to move.
6. Once your saddles are set correctly on a given bass, you most likely will never have to touch them again, ever, unless your playing style changes or you make a radical change in string gauges.
7. If despite correct neck relief you are still buzzing on the low frets, the nut slots may have been filed too deep. Consult a knowledgeable luthier to build up the slots or replace the nut.
8. If your relief is different between the low strings and the high strings, your neck is twisted. Raise the action enough to compensate, and if it's playable, then play it anyway. If it's not, then consult a knowledgeable luthier or replace the neck or bass. (Chances of this happening on a Dingwall are probably near zero. Every one I've seen has been dead perfect.)