Using the Non-MFC methods I set the number format to three places, change the font to bold and set underline.
Code:
IDispatch *pXlRange1;
{  
VARIANT range;
range.vt = VT_BSTR;
range.bstrVal = ::SysAllocString(L"C1:C3");
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYGET, &result, pXlSheet, L"Range", 1, range);

pXlRange1 = result.pdispVal;
}


{  
VARIANT frmt;
frmt.vt = VT_BSTR;
frmt.bstrVal = ::SysAllocString(L"0.000");
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYPUT, &result, pXlRange1, L"NumberFormat", 1, frmt);


}



IDispatch *pXlFont;
	{
VARIANT result;
VariantInit(&result);

AutoWrap(DISPATCH_PROPERTYGET, &result, pXlRange1, L"Font", 0);
pXlFont = result.pdispVal;


	}


	{
VARIANT result;
VARIANT xprop;
xprop.vt=VT_BOOL;
xprop.boolVal=TRUE;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYPUT, &result, pXlFont, L"Bold", 1,xprop);
	}

	{
VARIANT result;
VARIANT xprop;
xprop.vt=VT_I4;
xprop.lVal=2;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYPUT, &result, pXlFont, L"Underline", 1,xprop);
	}