§121.619 Alternate airport for destination: IFR or over-the-top: Domestic operations.
(a) No person may dispatch an airplane under IFR or over-the-top unless he lists at least one alternate airport for each destination airport in the dispatch release. When the weather conditions forecast for the destination and first alternate airport are marginal at least one additional alternate must be designated. However, no alternate airport is required if for at least 1 hour before and 1 hour after the estimated time of arrival at the destination airport the appropriate weather reports or forecasts, or any combination of them, indicate—
(1) The ceiling will be at least 2,000 feet above the airport elevation; and
(2) Visibility will be at least 3 miles.
(b) For the purposes of paragraph (a) of this section, the weather conditions at the alternate airport must meet the requirements of §121.625.
(c) No person may dispatch a flight unless he lists each required alternate airport in the dispatch release
Per the regs you are good up to 3SM, however alot of dispatchers would consider 3SM alternate adding territory.
For practical purposes you might consider that no alternate is required, but for anything meeting 1-2-3 in the 0200Z line would be smart to have fuel planed to make an alternate, even if not listed as such.
In actual dispatching you will also have an array of radar and weather charts at your disposel to help make descions on to add or not to add an alt/additonal fuel.