What level of portrait experience do you have?
Do you know how to light a portrait shot?
Do you own any off camera lighting?
Do you mean single subject portraits, full length, head and shoulders, groups?
Indoor/studio or outdoor?
The answers to these questions will point out that "portrait" photography is about the desired look. Perspective is probably the most important quality, but you have the right focal lengths covered already. You lack the ability to isolate the subject from the background as you have slow kit glass, but if you're shooting indoor/studio that won't matter because most strobe work is done F8 anyway.
Think more, it's not as simple as "what lens". If you have limited experience with potraits I wouldn't buy any glass at all - make sure you have lighting down first, even if it's some reflectors only.