syringes measure volume.

a 3cc syringe will hold 3ml. how many mg is 3ml? well, that depends on the concentration of what you're shooting. if substance is 250mg/ml then you need 1ml (or 1cc) to get 250mg.

1ml x 250mg/ml = 250mg

if its 100mg/ml then you need 2.5cc

2.5ml x 100mg/ml = 250mg