Yes, same happened with my DS at about 8 months - he started drinking only 3 or so ounces, maybe three or four days in a row, from his 11am and 3pm bottles. Then a few days later even started cutting down to 6oz at his 7am and 7pm. I dropped the 11 and 3, instead giving milk right before his nap at about 1pm, and he started taking the full 8 ounces from the three bottles. I would wait until your LO starts decreasing the amount s/he takes in the mid-day bottles and then combine them into one.
HTH