Manage disk partitions

Can I safely join partitions 3 and 5? Or remove partition 3?

No. Partition 3 is an extended partition, which holds the logical partition 5.

Can I add the free space on the right to them?

This free space represents 1.1 MB, which is virtually nothing. Because partition sizes are restricted by some alignment rules, it may not be possible to expand partitions, but again: it is not worth the trouble.

Partition 1 is of type W95 FAT32, and the contents are FAT (32-bit version) — Not Mounted. Any idea why this àrtition was created for?

You probably did not erase the full disk and have the Ubuntu installer repartition the entire drive. This Partition 1 probably is a leftover from earlier installations, and may also have been an EFI patition, which currently is not used.