I guess you are a Contractors Supply outlet who sells supplies for do it yourselfers’ and if they decide not to, you provide installation. What i am trying to figure out is why you need to manually calculate PST in the third journal, unless it is for employee purchases where you must recover the PST on PST Exmept Goods. Or you have bought items at a wholesaler PST exempt, that you do not resell and must recover the PST owed to the province.
If it only a matter of placing all transactions in one journal, I would use alphanumeric Reference Numbering , eg.
C1234570 etc. which I was able to do in NV1
A- Supplies only
B- Supplies & Installation
C- Sub-contractor, employees and Cash sales (or maybe even refunds & restocking charges).