Question -
Answer -
Suppose f is not one-one.
Then, there exists two elements, say 1 and 2 in the domain whose image in the co-domain is same.
Also, the image of 3 under f can be only one element.
Therefore, the range set can have at most two elements of the co-domain {1, 2, 3}
i.e f is not an onto function, a contradiction.
Hence, f must be one-one.