Please explain in what report sections the subreports are located. How are they linked to the main report (if they are linked)?
You say that there are multiple totals within each subreport--what are they? Are you expecting to get multiple grand totals or just one? This is hard to visualize.
You can pass shared variables to a section in the main report below the one in which a subreport is located, so you could create a shared variable in each sub like this (changing the name of each variable, e.g., to y, z):
whileprintingrecords;
shared numbervar x := sum({table.amt});
Then in the main report (lower section), you could add a formula like this:
whileprintingrecords;
shared numbervar x;
shared numbervar y;
shared numbervar z;
x+y+z
-LB