Huh...never knew at 6 months they still ate at 3 times per day. We've been feeding our 6 month old lab mix twice a day since she was 4 months old. She got 1.5 cups in the morning and 1.5 at night. She now gets 1 cup in the morning (roughly around 7:30am) and 1 cup at night (around 6:30pm) and hasn't had any issues with it. Shes 32 lbs and at the perfect weight according to our vet. We were told at 6 months, she should be eating between 1 1/2-2 1/4 cups per day so she gets 2 cups per day.
I'd just cold turkey move her to two meals a day. She will catch on quickly that she no longer gets fed middle of the day.