Summing every three values in row

[sambridge]

Hi. I hope someone can help me. I am going around in circles. I can create a manual formula which I have to update each month, but I want to automate once I select a new month.

I have a spreadsheet which has columns of data per month. I want to sum the year to date totals for ordinary time, time and a half and double time. I have used the index function for the month however as the year progresses I want to track the year to date figure.

I hope my note has not confused the issue to much and trust that the spreadsheet is more self explanatory than my words. In essence for the Month of October, the year to date std time hours should be the sum of cells D10, G10, J10 and M10.

Can anyone help with a formula in essence to sum every third column which expands and the year progresses.