We currently use multiple web servers accessing one MySQL server and fileserver. Looking at moving to the cloud, can I use this same setup and attach the EBS to multiple machine instances or what's another solution?
I would suggest using Amazon Elastic File System (EFS), which is designed to be able to attach with multiple EC2 instances and sharing the storage.
The answer to your question would be NO. Let me explain, as per EBS docs you can only attach EBS to one instance at a time. But attaching them with multiple instances will leave in the situation where you use one hard drive for two computers which is totally impractical.