18Then after three years I went up to Jerusalem to visit Cephas and remained with him fifteen days.
Paul is referring to three years after his conversion/meeting with Jesus Christ on the road to Damascus. It is not clear if three full years are meant since Jewish counting was inclusive at the time.1 If his conversion was in 31/32 AD, then his visit to Peter in Jerusalem took place in AD 33/34.2
18 Then after three years I went up to Jerusalem to visit Cephas and remained with him fifteen days.