If this is not the correct place to add this lease let me know where to move it. Thanks.
I found very little info out there for the Asterisk setup needed to make calls to a 3CX so once I got it working I wanted to share in case anyone else is looking for the same info. There are probably better ways to set this up, but it is working so I left it like this. 
IPs and Passwords have been changed in the setup provided below.
This is the setup I used to get a FreePBX 15 - Asterisk 16.17.0 registered and calling to a 3CX.
Calls are generated from a custom script that creates .call files.
; My SIP peer on Asterisk that is used to call the 3CX is 1001
; sip_iwatsu.conf
[1001]
type=peer
username=dkjhts74
fromuser=1001
context=alert
host=10.12.6.3 ; 3CX IP
secret=gjrefds8ue5
qualify=yes
fromdomain=10.12.6.3 ; 3CX IP
transport=udp
nat=auto
dtmfmode=auto
; sip.conf
[general]
externip=114.137.55.8 ; Public IP of Asterisk
nat=force_rport,comedia
transport=tcp,udp
tcpenable=yes
tcpbindaddr=0.0.0.0
udpbindaddr=0.0.0.0
allowguest=no
allowoverlap=no
alwaysauthreject=yes
srvlookup=yes
disallow=all
allow=g722
allow=ulaw
allow=alaw
accept_outofcall_message=yes
auth_message_requests=yes
register =>1001:gjrefds8ue5:dkjhts74@10.12.6.3:5060/1001