I have to disagree with the person who told you that. "7 fish" is extremely vague. A ten gallon could hold 7 bettas, at a push, or 7 tetras, but platies have a relatively large bioload and a 10 gallon is fully stocked with 4 platies, nothing else.
Also, as you have 3 platies, it sounds like you are proposing a four-female sorority. That is extremely risky - sororities are not stable groups and at least 5 is recommended for the security of the fish. In a 10 gallon, the safest thing is to have 6 females and nothing else.
Basically, a 10 gallon just doesn't have the capacity, in terms of both space and bioload, to safely house both your platies and a sorority.
Honestly, I'm not trying to be mean or critical, I just want to be helpful. :)