You have two separate problems here.
a) 3 for $5 needs to be expressed as cost per unit. Write $5/(3 units), and then simplify. Here the unit rate is ($5/3) / unit, or $1.67 per unit.
b) 4 for $6: Write $6/(4 units), and then reduce to lowest terms:
(6/4) dollars per unit, or $1.50/unit