Doc generation / Merge Pdf

Hello,
I am encountering difficulties generating a document via an omniscript.
I have 3 templates that I would like to merge into a single pdf. These are conditional on certain fields of a quote (proposal, annexes, conditions).
I manage to generate the 3 corresponding templates, then perform the merge in a second step within a single PDF.
So I have a step to generate the 3 documents, and a step to generate the merged document.
If possible, I would like to generate the 3 documents on the server side, and only display the final document to the user.
Are there any ideas to achieve this?