I have some information to share with you about being able to add three suppliers for one product on the same product record in QuickBooks Online.
At this time, the ability to add three suppliers in one transaction in the same product record isn't available for us. We can only attach one vendor per transaction. However, you have the option to append more than one product to the vendor transaction (a bill, P.O).
On the other hand, QuickBooks allows you to add multiple customers for one transaction, this can be done by adding a sub-customer. To give you detailed information on how this works, you may read through these references below:
Also, we are striving hard to make our products convenient for our users. This is why QuickBooks Online is constantly evolving and adding features based largely on the preferences received from our customers. For now, I'd suggest visiting our QuickBooks Online Blog to be updated with our latest news and improvements.
I've added here a couple of resources that you can visit about some "How do I" questions in QuickBooks Online for your future use: