Is it ok that a preemie currently in his 5months to drink as much as 120ml each 3hourly feed?

I think it is quite an average amount. One of my friends told me that her baby was drink around 120-150ml at 5 months (adjusted), every 3 hours or so.

120ml 3 hourly is not counted a lot, there is babies drinking up to 150-180ml. As long your baby didn't vomit or cry the feed, should be ok.