Question
im doing a science project and i need to know the 3 most important fertilizers i should use. can you help me?

Answer
There are actually 20 minerals needed for plant growth.  Six 'macronutrients' are needed in the biggest amounts: Nitrogen (N), Phosphorus (P) and Potassium (K) head the list and are probably the ones you are asking for.  Calcium (Ca), Magnesium (Mg) and Sulfur (S) are needed in smaller amounts.  The rest are needed in tiny, trace amounts and are called 'micronutrients'.  A shortage of ANY of these will make a plant sick -- or kill it.

Technically, however, you are asking about the 3 most important fertilizers.  An organic gardener would answer this question a little differently.  Although plants need the same 20 minerals, you would not 'fertilize' them with N, P, K etc.  Instead, you would put something good in the Soil that would deliver N, P and K etc to the plant while it decomposes.  That 'good' material can be Compost, Manure, Fish Emulsion, Bloodmeal, Bonemeal, Earthworm Castings, or a long list of other things that are not fertilizer but still contain the critical N-P-K that plants need to thrive.

Anything else?

THE LONG ISLAND GARDENER