Hi.
I'm encrypting my backup drives(external HDD's) with Veracrypt.
I let Veracrypt use Ext3 which it offered as default i think.

Current backup software used is Timeshift and Backup Tool-both provided in the linux Mint OS of the system being backed up.

Eventually the backup drives will be used for the other PC's also which are also running GNU Linux-two more, different OS's

Is there any issue using Ext3 in this case, or should i use Ext4 and if so why?

I'd use 4 if possible, always.

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If you know that someday you will need to access the content of that filesystem with something that does not support EXT4, then you need EXT3. Some other operating systems that do support EXT3, and Linux kernels much more than two decades old, don't support EXT4, and some that aren't quite so old don't support all EXT4 features, the most significant of which is the 64bit option needed by the largest filesystems. IOW, EXT3 is arguably more widely supported. A good compromise for some is to use EXT4 with the 64bit option disabled. As long as you're sticking with recent Linux distros, you should feel confident with the more evolved EXT4 version, and can enjoy better performance, including support for larger maximum size volumes and files. I still have lots of installations using EXT3, probably around half of all I have.

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