The last parameter should be a pointer to a TBBUTTON structure.
And the message you send retrieves the toolbar button's details through that structure.
Eg
TBBUTTON tbBtn = new TBBUTTON();
SendMessage(hToolbar, TB_GETBUTTON, i,
tbBtn);
The TBBUTTON structure holds some info like it's state/style etc.
Check the msdn for more details on the structure & usage.
Your interop definition for the function would be
[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int SendMessage(IntPtr hWnd, int msg, int wParam, out TBBUTTON tbBtn);
(you'll have to define the TBBUTTON structure as well)