Don't think in fixed terms, but in variable terms
define
_dtc = month(CurrentDate)
_dtm01 = month(DateAdd("m",-1,CurrentDate))
_dtm02 =  month(DateAdd("m",-2,CurrentDate))

these will be headers, and today April 17th
_dtc = 4
_dtm01 = 3 as in minus 01 months
_dtm02 = 2 as in minus 02 months

then, check your table against those
Remember to SELECT for records in past year, since this in only looking at the month!

if month({table.invoicedate}) = {@_dtc}
  then
     monthc := monthc + {table.invoice}
     whatever you want to do; accumulate, etc..
if month({table.invoicedate}) = {@_dtm01}
  then
     monthm01 := monthm01 + {table.invoice}
     whatever you want to do; accumulate, etc..

and so on; you could probably also do a CASE statement

And for your report, use the _dt variable across the top, with the respective monthc or monthm variables in your report under those columns.