Group By clause Error

My columns are CustId, Fiscal Year, and this calculated field. That is all I am wanting. However, I keep getting an group by error for InvoiceDate. How do I get around this?

case
when Invchead.FiscalYear = datepart(year,Constants.Today)and invchead.invoicedate=dateadd(year, -3,Constants.Today) then (sum(invcdtl.ExtPrice))
when Invchead.FiscalYear = datepart(year,Constants.Today)and invchead.invoicedate=dateadd(year, -2,Constants.Today) then (sum(invcdtl.ExtPrice))
when Invchead.FiscalYear = datepart(year,Constants.Today)and invchead.invoicedate=dateadd(year, -3,Constants.Today) then (sum(invcdtl.ExtPrice))
else (sum(invcdtl.ExtPrice))
end

What error are you getting?

Telling me that InvcHead.Invoicedate is not in a group by clause

Changed to this due to errors in the original. My end game is to have a simple three column which will show my the YTD absed upon todays date for the past thre years. So I want to see the YTD for everything 03/15/2015 and earlier in the same fiscal year, 03/15/2016 and earlier in the same fiscal year, 03/15/2017 and earlier in the same fiscal year, and 03/15/2018 and earlier in the same fiscal year.

case
when Invchead.FiscalYear = datepart(year,dateadd(year, -3, Constants.Today))and invchead.invoicedate<=dateadd(year, -3,Constants.Today) then (sum(invcdtl.ExtPrice))
when Invchead.FiscalYear = datepart(year,dateadd(year, -2, Constants.Today))and invchead.invoicedate<=dateadd(year, -2,Constants.Today) then (sum(invcdtl.ExtPrice))
when Invchead.FiscalYear = datepart(year,dateadd(year, -1, Constants.Today))and invchead.invoicedate<=dateadd(year, -1,Constants.Today) then (sum(invcdtl.ExtPrice))

else (sum(invcdtl.ExtPrice))
end