Time signatures occasionally do away with the bottom number. While I tend to see this more often in modern music, the tradition goes back centuries (in fact, to before time signatures were actually a thing).
If I understand your question correctly, you have this:
Which tells you that there are three "beats" in the measure, and it's up to you to determine the denominator of the key signature. The above measure is equivalent to:
Perhaps a clearer notation would be:
Although one measure of 6/4 has the same note durations as 3/2, 3/2 has three large beats in the measure instead of the two beats that 6/4 has. (It's just like the distinction between 6/8 and 3/4.)
Since you're given a numerator of 3 telling you there are three beats in a measure, this is 3/2.