What is the recipe to produce a 3M solution of Sodium Nitrate?
You can produce 3 Mol sodium nitrate by neutralizing 3 Mol of nitric acid with 3 Mol of sodium carbonate:

HNO3 + NaHCO3 → NaNO3 + H2O + CO2

Or 3 Mol of sodium bicarbonate:

2 HNO3 + Na2CO3 → 2 NaNO3 + H2O + CO2