I to find the equivalent ratios

You want the second number of all your ratios to be 3.
Divide the second number by 3, then use that quotient to divide both numbers.
12 : 9 ⇒ (12/3) : (9/3) = 4 : 3
24 : 12 ⇒ (24/4) : (12/4) = 6 : 3
3 : 1 ⇒ (3×3) : (1×3) = 9 : 3
28 : 21 ⇒ (28/7) : (21/7) = 4 : 3
40 : 24 ⇒ (40/8) : (24/8) = 5 : 3
36 : 12 ⇒ (36/4) : (12/4) = 9 : 3
48 : 18 ⇒ (48/6) : (18/6) = 8 : 3
28 : 12 ⇒ (28/4) : (12/4) = 7 : 3