In replication A->B->C, can A be binlog_format=STATEMENT and B be binlog_format=ROW?
STATEMENT &rarr; ROW is absolutely a valid configuration.

The thing you've probably overlooked is that Server B needs log_slave_updates enabled, or it won't replicate anything from A to C.



But to address the question at hand, consider the following table, which describes the comparison between configured @@binlog_format and what is actually written to the server's binlog.  Assume A is master, B is slave of A,  no changes are made directly to B, and of course log_slave_updates is enabled on B:

A binlog_format | A actually writes | B binlog_format | B actually writes
-------------------------------------------------------------------------
STATEMENT         STATEMENT           STATEMENT         STATEMENT
STATEMENT         STATEMENT           MIXED             MIXED
STATEMENT         STATEMENT           ROW               ROW

MIXED             MIXED               STATEMENT         TOTALLY FAILS EVENTUALLY
MIXED             MIXED               MIXED             MIXED
MIXED             MIXED               ROW               ROW

ROW               ROW                 STATEMENT         TOTALLY FAILS VERY QUICKLY
ROW               ROW                 MIXED             ROW
ROW               ROW                 ROW               ROW


What's going on here?

When B writes to its binary log, it writes an entry to replicate the change it made.  It writes these entries in the format its local optimizer determines should be used.  When STATEMENT or ROW mode is in effect, that's the only choice, so incoming STATEMENT becomes outgoing ROW, in your case.  Server B easily translates because it executes the incoming event and then simply logs which rows it inserted/updated/deleted in its local data, and the values of the columns in those rows.


  Each MySQL Server can set its own and only its own binary logging format (true whether binlog_format is set with global or session scope). This means that changing the logging format on a replication master does not cause a slave to change its logging format to match. [...]
  
  Changing the binary logging format on the master while replication is ongoing, or without also changing it on the slave can cause replication to fail with errors such as Error executing row event: 'Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT.'
  
  http://dev.mysql.com/doc/refman/5.7/en/binary-log-setting.html


That's what I was referring to, above, with TOTALLY FAILS.  The only invalid configuration is for a server to be in STATEMENT mode while at least one upstream master is in ROW or MIXED.

Why?  Every possible query that can be represented in STATEMENT format can always be subsequently represented in ROW format, because ROW format is absolutely deterministic.  Conversely, no query replicated in ROW format can ever be subsequently replicated in STATEMENT format.

Note that MIXED format simply gives the local optimizer the option on a query by query basis of how to replicate -- STATEMENT or ROW -- but if the incoming replication event is ROW the only outgoing choice is also ROW.



Also, STATEMENT mode is seriously terrible.  Get away from it as soon as you can.  Before then, it's vitally critical to verify that the data on B is identical to the data on A in every way, because the data on B can slip out of sync much easier and go undetected when A is in STATEMENT mode.