Grub2 problem - can only boot to 2 of 3 OS's

[jim]

I had a system with Mint18.3 and Ubuntu18 installed. I had 100GB of free 
space at the end of the disk. I decided I wanted to install mint19.3 in 
the free space. I booted from the Mint19.3 install dvd. I used gparted 
to shrink the Ubuntu partition so I ended up with about 195GB of free 
space. I then had gparted create a new partition in the free space and 
installed Mint19.3.

This is my configuration:

Partition 1 = sda1 - Ubuntu18

Partition 2 (extended) = sda6 - Mint18.3
                          sda5 - swap

Partition 3 = sda3 - Mint19.3

When the Mint19 install rebooted the first time I noticed the grub menu 
was visually much larger looking than I normally saw under Mint18.3 but 
I didn't think much of it. When I was done working with Mint19.3 I 
restarted the system when the overly large grub menu cam up the only 
options were Mint19.3 & Ubutu18, Mint18.3 was not shown.

I booted to Ubuntu and did some googling and read the solution was to 
run sudo update-grub2. I ran it. As the output scrolled by I did see 
Mint18.3, but when I rebooted Mint18.3 still was not an option. I booted 
to Mint19.3  and ran the sudo update-grub2 command but still Mint18.3 is 
not an option in the grub menu.

It's been a while since I have installed a fresh OS so what do I need to 
do to get Mint18.3 to show up in grub. I upgraded Ubuntu16 to 18 so grub 
was not involved there.

Thanks, Jim