Manjaro won’t install with Windows 7 + Windows 10 already in dual boot.
I have installed Windows 7 in UEFI and Windows 10 also in UEFI.
When I try to install Manjaro it sometimes fails on grub installation and sometimes it finishes but when I reboot it just boots back to Windows Boot Manager. Under Windows 7 with Partition Guru it says the Partition it installes is ext4.
Your configuration is highly unusual with a dual-boot Windows 
I highly doubt that - unless of course you are a super user who knows how to circumvent the limitations of the Windows 7 installer (the installer does not support creating a GPT filesystem) - and Windows 7 is 99.9% a BIOS/MBR installation.
I use a HP Probook 4730s. You can install Windows 7 in UEFI if you use a DVD. If you go to the boot menu on my laptop you can choose to boot from an efi file. I clicked on it chose the dvd drive and found the efi file on my win 7 installation dvd. I booted from it and I was able to install Windows 7 in UEFI mode with Windows 10 in dual boot.
Here is a Screenshot from disk managment.
The 100GB UEFI partition is the partition where I installed Manjaro before manjaro installation it was unformatted.
I’m not familiar with dual boot setups but one thing to check is that that 100 MB UEFI partition has enough space for the 3 OS’s you want to boot.
Should I make the partition bigger? And if how?
If needed I would delete Windows 10. I used it mainly for performance testing and work till I figured how to get Teams properly working on Win 7.
That depends how full it is, there should be a tool for that on any of the windows systems you have running.
I would dump windows 7, you will have issues running new software on it, there are no more patches for bugs/security issues.