Such an Excel sheet is easy to do.
It contains -say- 5 columns.
column1:
the name of the input or output stream (solid or gaz)
you are the only person able to make this list
column2:
you indicate there if it is an input or an output stream
column3:
the flow of each stream in ad hoc units (t/h, Nm³/h, ...)
column4:
the SO3 analysis for each stream,
you get them from the lab for the solids,
and from gas measurement for the gases
(some unit conversions might be needed)
column5:
the SO3 stream in t/h
you calculate this by (SO3 analysis) * (flow) * InOrOut
where InOrOut is +1 for inputs and -1 for outputs
Once this is done, you sum up all the numbers in column5 and your balance is done.
Most of the work will be in measurements.
Eventually some measurements will not be possible, in which case you need to get the data by indirect measurements. Of course, these difficulties are different in in situations.