I think you just use the Ampersand --
Highest: Sum(IIf([NET_DUE_DATE] Between Date()-31 And Date()-90,([G/L Principl Amt])*0.25,0)) & " " & Sum(IIf([NET_DUE_DATE] Between Date()-91 And Date()-180,([G/L Principl Amt])*0.5,0)) & " " & Sum(IIf([NET_DUE_DATE]<=Date()-180,([G/L Principl Amt])*1,0))...