Mounting should be vaguely similar to the SMB1 mounting you had before.

Download and install SMB Client 3, and “Network Authentication Service” (aka kerberos 5) from here:

https://www-01.ibm.com/marketing/iwm/iwm/web/pickUrxNew.do?source=aixbp

Ensure your Windows 201x server has SMB v3 enabled.

You want a service account in AD to use for your SMB3 mounts on AIX.

Notes about options:

encryption should be desired and secure_negotiate should be desired.
signing should be enabled
​​​​​pver should be 3.0.2
The kerberos realm specified in the “wrkgrp” option must be in all UPPERCASE if your domain is in uppercase.
The username provided for mounting is used for all read/write permissions/access.  
UID and GID default to root.system, but you can specify others.
fmode is the inverse of umask, and what the files’ permissions look like across the whole share.  Default is 755.
port can be 139 (ipv4) or 445 (ipv4 or ipv6).  Default is 445.

/etc/filesystems format:

/mnt:
     dev = /corpshare
     vfs = smbc
     mount = true
     options = “wrkgrp=CORP.DOMAIN,signing=enabled,pver=3.0.2,encryption=desired,secure_negotiate=desired”
     nodename = win2016server.corp.domain/sambauser

Command line example

mount -v smbc -n win2016server.corp.domain/sambauser/Passw0rd! \
-o “wrkgrp=CORP.DOMAIN,port=445,signing=required,encryption=required, \
secure_negotiate=desired,pver=auto” /corpshare /mnt

Store the samba credentials

mksmbcred -s win2016server.corp.comain -u sambauser [-p password]

See also lssmbcred, chsmbcred, and rmsmbcred.

Reference 2021:

https://www.ibm.com/docs/en/aix/7.2?topic=protocol-server-message-block-smb-client-file-system