Baby should be fine
3ml =45mg of Zantac
) and it maybe a bit high for your baby, but should be fine.
In adult, we often give double the dose for single-daily dose (i.e. 150 twice/day, or 300 once daily). You can skip the next dose and i would not expect any problem. See my previous answer about dosage of Zantac (ranitidine) for baby--a weight-base calculation. If you have additional concerns, consult doc. Good luck.