Unless, you are dedicating resources on the virtual machine, it is unlikely that will perform as well as 3 smaller physical machines. In fact, you will incur a 10% performance penalty for indexing by simply running virtually (worse if there is contention for CPU, RAM, or disk).
Also, a single indexing pipeline would use 4 CPUs per machine, thus 3 servers would have 12 CPU worth of indexing horsepower. As of 6.3 you can have up to 2 (max recommended) indexing pipelines which would give you 8 CPU worth of indexing horsepower on the "big server", which would be less than the 3 smaller servers, but would leave more CPUs available for handling search. If that meets your needs, it's probably fine.
Personally, if I was setting this up, I would want the 3 servers with known dedicated resources, and the inherent redundancy that is associated with it. But if you are going for ease of use and simplified maintenance/administration, 1 server does fit the bill.
Info on indexing parallelization:
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