Why are you considering using three databases for a simple application? Start with one, and break specific domains out if and when you need to.
Scalability and speed are not inherent attributes of NoSQL databases. Many of them are designed with an eye to both, but it's not to be taken for granted. Something like Cassandra, for example, would be very bad for your application because it's optimized for high write volume and you're mostly reading. HBase would be bad because you can't do ad-hoc queries easily. Meanwhile, relational databases can scale well enough until you're Amazon or Google tier (even Instagram runs on Postgres), and designed correctly they're plenty fast.
Last, if you're considering MySQL you should think about Postgres; they're in the same weight class, and I would use Postgres over MySQL in a heartbeat. Overkill only matters when you add unnecessary complexity, such as for example splitting your data layer across three independent DBMSs before you've even gotten anything off the ground.
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