It is impossible not to have a set with 5 cards.
You can make a set with 3 cards of the same color, or 3 cards of different colors.
For instance with:
2 red
2 blue
if the 5th is red, you have a 3 cards red set,
if the 5th is blue, you have a 3 cards blue set,
if the 5th is green(or black), you have a multicolor set: 1 red, 1 blue, 1 green(/black).
It is impossible not to have a set with 5 cards.
You can make a set with 3 cards of the same color, or 3 cards of different colors.
For instance with:
2 red
2 blue
if the 5th is red, you have a 3 cards red set,
if the 5th is blue, you have a 3 cards blue set,
if the 5th is green(or black), you have a multicolor set: 1 red, 1 blue, 1 green(/black).