Sounds like a project that I'm just finishing up and getting ready to market for rent. We purchased a distressed triplex that needed a substantial amount of work. Vandals had stolen all the copper, ACs, appliances, etc. Also needed new drywall, paint, carpet, and the list goes on.
We had all 3 units rehabbed at the same time and am glad that we did it that way. Not only did we save money and time, but all the work going on drew a lot of attention to our property. Several neighbors stopped by the see what was going on and we also got several leads for potential renters.
We could have worked on one at a time and then tried to place a tenant while the other 2 units got worked on, but felt the tenants would have been dissatisfied with all the noise and such going on next door. Plus it's also nice to provide your future tenants with the option to choose which unit they would like best. First come first serve :)
If I was a potential tenant and I came to look at my property where one unit looks great, but the others look liked a crime scene, that would most likely have a negative impact on how I feel about renting that property. However, if I showed up to view a property and all the units were all fixed up and looking top notch, I would probably have a better feeling about renting here.