Quick answer
Son las tres · Es la una
The hour is feminine and plural, so it takes son las — except one o’clock, which takes es la.
Ser, and the hidden noun
The hours are feminine because las horas is understood behind them.
Minutes forwards and backwards
Up to the half hour, add with y. After it, subtract from the next hour with menos.
At what time
The clock reading uses ser. The appointment uses a las.
De la mañana, de la tarde, de la noche replace a.m. and p.m., and they only appear with a stated hour.
Memory trick
Only one o’clock is singular. Every other hour is plural, in both son las and a las.
Typical mistakes
Es las dos.
Son las dos.
Dos is plural, so the verb is son. Only la una takes es.
Está la una.
Es la una.
The time always uses ser, never estar, even though it describes a passing moment.
En las siete llego.
A las siete llego.
The preposition for a point in time on the clock is a, not en.
Check yourself
___ la una y media.