I am not entirely sure I get your question, the whole idea of an LCA calculation, as openLCA performs, is to be able to get the results of the entire system, with multiple processes and flows normally. If the three processes are somehow independent and not connected, as maybe in your case, you can create a process ("my final process" or so) where all the three products from these processes are input, and calculate this. This will give you the result for all the processes and their flows.
All the best,
Andreas