No idea why you can't connect with SMB3 - we have not had any one else with the issue as far as I am aware.
Note also that things are moving fast. There is an update to Samba so we are now using the upstream version 4.10.16
yum --enablerepo=smeupdates-testing update
or
yum --enablerepo=smeupdates-testing install e-smith-samba
That should install the new versions, but note you might still errors with some things - ntpd is not working correctly right now:
rpm -qa |grep samba
samba-client-libs-4.10.16-7.el7_9.x86_64
samba-common-4.10.16-7.el7_9.noarch
samba-winbind-modules-4.10.16-7.el7_9.x86_64
samba-client-4.10.16-7.el7_9.x86_64
e-smith-samba-2.6.0-12.el7.sme.noarch
samba-common-tools-4.10.16-7.el7_9.x86_64
samba-common-libs-4.10.16-7.el7_9.x86_64
samba-dc-libs-4.10.16-7.el7_9.x86_64
samba-python-4.10.16-7.el7_9.x86_64
samba-libs-4.10.16-7.el7_9.x86_64
samba-winbind-4.10.16-7.el7_9.x86_64
samba-dc-4.10.16-7.el7_9.x86_64
samba-4.10.16-7.el7_9.x86_64
Check these templates for the max/min settings and so you can see how it works.
/etc/e-smith/templates/etc/smb.conf/11maxProtocol
/etc/e-smith/templates/etc/smb.conf/11minProtocol
Unless you override, SME will use SMB3.