I think you have to come up with something that covers the return policy of all three dropshippers. Since it would look quite odd (to your customers) if you have 3 different return policies.
Also, since these dropshippers are located in different places - it might be wise to mention that they (your customers) must email you prior to returning any of the items so you can provide the correct return address and at the same time inform your dropshipper.
Do their return policies vary too much? Hopefully for your convenience they're the same or quite similar.
Hope this helps. :)