Simple - Set range as first empty row in range

Hi All,

I have a real simple one that I think I'm making harder than it really is.

I have a sheet where I have several shared users using a userform to enter data.
To prevent them overwriting each other, I want to give each username a range. John is rows 3-22, Mike is rows 23-42, Mary is rows 43-62..
Then I want it to find the first empty row within that range
For instance:

Dim NextRow As Long
if Application.Username = "John" Then
NextRow = Range("Sheet").End(xlDown).row + 1

The only thing I'm missing is how to specify the row to start on. The above code just finds an empty row, what do I add to make john start at row 3?

Thanks!