Answer is "it depends"... on server spec, server load, usage of each functionality.
Only you can know what are the requirements for each individual item - based on that you need to determine the impact of having them on the same server based on the spec of the server
As an example
3 servers
1 - SSAS -- 4 cores 20 GB ram - normal usage 30% cpu, 5GB ram
2 - SSRS -- 4 cores 20 GB ram - normal usage 10% cpu, 15GB ram
3 - SQL -- 4 cores 20 GB ram - normal usage 10% cpu, 15GB ram
Moving the 3 servers onto a single server
serv1 - 4 cores 20 GB - would be overloaded on memory - cpu acceptable
serv2 - 2 cores 60 GB - would be overloaded on cpu - memory acceptable
serv3 - 8 cores 160 GB - would be quite happy