1. 'Common people' could be misunderstood. 'Common' can mean crude and unsophisticated, and it has connotations of class hierarchy - royals and aristocrats in the past would refer to the masses as 'the common people'. It's best avoided. 2. 'Average' is a statistical term, and we tend to use it in the singular. We talk about 'the average person', in the singular, as an example of a typical middle-class, middle-income citizen with nothing remarkable about them. 3. 'Ordinary people' is probably your best option. Or, as Sai suggests, you could use 'normal' if you are suggesting that there is something 'abnormal' about celebrities. For example, if you are talking about the unreasonable way that some celebrities behave, you can refer to 'normal people' as a contrast.

They all work, and they’re all synonymous. Some may be suited better for a situation than an other; you'd have to tell us how you're using it. The one I hear most out of those three would be "average people." But, the one I hear used most is not on the list... And that's "normal people."

All of these work. Which one is best to use, depends on why your using it.

Ordinary people, I think. Maybe you can use "ordinary folk". The word "folk" means families, relatives and friends.