Replication with 9.4

Hi,

I would like to fix a issue I'm facing of with the version 9.4 streaming
replication.
is it possible to set on the fly the synchronous commit on the master
(or standby?)
which only sync commit the hot standby node used by the client who has a
read only sql session on?
example:
node1 node2 node3 are hot standby with replication slots (physical) on
master 1
node1 has the priority 1 since it's first on the names list
now a client open a ready only session on node3 and start a transaction
update but
within this transaction a sql select is done on the hot standby from the
updated table

thanks

Franck