I have 3 servers each with a SQL Server 2017 instance, and 3 different applications that connect to each sql server instance, basically:
Application 1 -> Instance 1 (SQL Server 2017)
Application 2 -> Instance 2 (SQL Server 2017)
Application 3 -> Instance 3 (SQL Server 2017)
Would it be a bad practice to put all the databases in a single instance?
There is nothing inherently wrong with hosting multiple application databases on one instance, but each application may have caveats. I think the most we can really advise on are personal experiences where this has become a problem.
For example, if the application is expecting a specific Login to exist with a specific password, but that password is different between application instances, you're going to run into an issue.
Also, if you determine one of the instances of the application benefits from an instance specific setting, but it also hurts the performance of one of the other applications, you're going to have some friction.
If you have the benefit of contacting a vendor to see if they have an opinion on this, it is probably well worth your time to do so.
This is OK to do. Just make sure the hardware can cope.
If each separate workload needs, say, 8GB RAM the combined instance will need (something like) 24GB. If each instance can keep 4 CPUs busy the combined instance will need (something like) 12 cores.
Replacing one 500GB drive with one 1.5TB will not give the needed IOPS. Consider the load on TempDB, too.
