How do you say it is 3 o'clock in the afternoon in Spanish?

las tres en punto

When referring to time in Spanish:

You either use

Es la...

or

Son las...

You ONLY use Es la...when saying: it is 1 o'clock etc.

And you use Son las...when saying: it's is 2 o'clock, 3 o'clock 4, 5, 6, (every number except 1)
'Three O'clock' is 'las tres de la manana' (morning) or noche (night).

'Las tres de la tarde' is 3p.m.