Create alias for ssh connecting

Use the intended way and write the options and aliases into ~/.ssh/config:

Host 1
  Port 12345
  User my_user
  HostName 123.123.123.1

Host 2
  Port 12345
  User my_user
  HostName 123.123.123.2

and so on...

And then connect just using

ssh 1
ssh 2
...

This calls for a function -- simple and robust, whereas an alias in this case would be fragile.

Something like this should do:

function ssht () {
    [[ $1 =~ ^(1|2|3)$ ]] || { echo 'Not a valid last octet value !!' && return ;}
    ip=123.123.123.$1
    ssh my_user@"$ip" -p 12345
}

The condition [[ $1 =~ ^(1|2|3)$ ]] makes sure you have entered one of 1, 2, 3 as first argument (any trailing argument is ignored).

Now, you can give the desired last octet as the first argument:

ssht 1
ssht 2
ssht 3

Put this in your ~/.bashrc for having it available in any interactive session.

You can use patterns in ~/.ssh/config. From man ssh_config:

PATTERNS
     A pattern consists of zero or more non-whitespace characters, ‘*’ (a
     wildcard that matches zero or more characters), or ‘?’ (a wildcard that
     matches exactly one character).  For example, to specify a set of
     declarations for any host in the “.co.uk” set of domains, the following
     pattern could be used:

           Host *.co.uk

     The following pattern would match any host in the 192.168.0.[0-9] network
     range:

           Host 192.168.0.?

Combined with:

HostName
    Specifies the real host name to log into.  This can be used to
    specify nicknames or abbreviations for hosts.  If the hostname
    contains the character sequence ‘%h’, then this will be replaced
    with the host name specified on the command line (this is useful
    for manipulating unqualified names).  The character sequence ‘%%’
    will be replaced by a single ‘%’ character, which may be used
    when specifying IPv6 link-local addresses.

So, in your ~/.ssh/config, put:

Host ?
    Hostname 123.123.123.%h
    Port 12345
    User my_user

Then:

$ ssh -v 1
OpenSSH_7.4p1, LibreSSL 2.5.0
debug1: Reading configuration data /home/muru/.ssh/config
debug1: /home/muru/.ssh/config line 41: Applying options for ?
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Connecting to 123.123.123.1 [123.123.123.1] port 12345.
debug1: connect to address 123.123.123.1 port 12345: Connection refused
ssh: connect to host 123.123.123.1 port 12345: Connection refused