Upvote:1
I am afraid this is normal, particularly if A, B and C are all in different countries. Essentially by doing a multiple destination trip (called a "circle trip") you are seen as someone more able to pay lots of money than someone doing a simple round trip; ergo, the price is higher.
If the airline has ruled that the very cheapest fares cannot be combined on to one ticket, then the vendor cannot do anything about it. They must be sold separately. It has to do with the fare construction and ticket rules.
Now it is very possible that the online travel agents are not finding the lowest possible price available for the itinerary on one ticket. You could have a play with http://matrix.itasoftware.com which is often a little better, but you would need to find a travel agent to book the flights as it is only a demonstration system.
In your shoes I would take quotes of the single ticket and the combined multiple tickets to your boss and ask which he would prefer that you buy.