What are you trying to get out of interviewing those people?
Personas should be based on qualitative and quantitative user research.
Quantitative studies (aiming at statistics, not insights): Test at least 20 users to get statistically significant numbers; tight confidence intervals require even more users.
Three groups of 4-5 people (max total of 15) might not be enough for correlated results.
the vast majority of your user research should be qualitative — that is, aimed at collecting insights to drive your design, not numbers to impress people in PowerPoint. The main argument for small tests is simply return on investment: testing costs increase with each additional study participant, yet the number of findings quickly reaches the point of diminishing returns. There's little additional benefit to running more than 5 people through the same study; ROI drops like a stone with a bigger N .
For qualitative results, 15 people will result in a lot of same answers.
You might want to consider the costs (money and time) for doing this research. For getting demographics and such you might want to consider a survey under 20+ people. For getting qualitative answers, i.e. "why do user use this service?", I would advice one-on-one interviews with 5 to 7 persons.
Focus groups are great for getting things like user needs by letting them do a card sorting exercise. But things like user needs should not be included in personas. Personas should describe who your user is and in what context (why/where) they use your service or application; not HOW they use your service or application.
That's my opinion. Hope it helps.