Continue series after a break

I'm writing a program to make projections. Part 1 looks at history, part 2 looks at future.

In part 1, Cells D2,E2, and F2 say "June, July, August" this month, next month they'll say "June, July, August, September". Following the months is the Total column. Then we come to part 2. To the right of the total column I'd like the continuation of the series for my projection (3 months only). So, for this month, G2 is Total, H2, I2, and J2 should be "September, October, November". Next month they all shift one to the right.

I know how to do the shift to the right, but how do I continue the series?

Any help is greatly appreciated.