otherwise with let's say three Linuxes I would need 6 partitions already and that wouldn't fit an extended partition!
If I'm understanding you correctly, you're operating under a misapprehension. An extended partition can hold an arbitrarily large number of logical partitions. The exact limit depends on the size of the disk, but for any modern hard disk, the limit far exceeds six partitions. I've created test disks with something on the order of 100 logical partitions and that works fine. (That said, some older Linux distributions max out at, IIRC, 16 partitions per disk. Ubuntu has ways around that, though, as do most modern Linux distributions.) Many modern computers use the GUID Partition Table (GPT) partitioning system, which has a default limit of 128 partitions, and that limit can be raised if necessary.
But I see that many people recommend almost as a necessity to have a separate /home partition.
I would see that as an advantage if it could be used for different systems at the same time or something like that, which is not possible. The home folder contains settings which are very system-specific and also program-specific and program-version-specific.
It's possible, and safe, to share a /home partition between distributions. The trick is to ensure that users have different home directories on that shared /home partition. For instance, if you're sharing the /home partition between Ubuntu and Mint, you might give the user Fred the home directory /home/fred-ubuntu under Ubuntu and /home/fred-mint under Mint.
This is most easily accomplished by giving users different usernames in each distribution -- thus, Fred might use fred-ubuntu and fred-mint, depending on which distribution is being booted. You can, however, use the same username (say, fred) across distributions, but assign different home directories. Ubuntu's installer doesn't make this easy for accounts created at system installation time, but it can be accomplished by either renaming the home directory or changing the username after installing the OS. (Both tasks can be done with the usermod command, although renaming the home directory will also require using mv.) Some distributions enable customizing the home directory name at system installation time, which is helpful when setting something like this up.
With the separate home directories created, you can create symbolic links to simplify access -- for instance, you might have a symbolic link from ~/mint to ../fred-mint in the /home/fred-ubuntu directory to make it easier to access Mint files from Ubuntu; or you could link subdirectories you use (like ~/Downloads or ~/Videos) directly.
Setting things up this way does take more initial effort than creating separate /home partitions for each distribution, or not using separate /home partitions at all; however, it can make for more efficient use of space.
Stepping back further, though, IMHO installing multiple Linux distributions in a multi-boot environment is unlikely to be very helpful. If you want to learn about different distributions (say, RPM vs. Debian package management systems), running one distribution in a virtualized environment (VirtualBox, VMware, QEMU/KVM, etc.) can be quite effective and will save you from multi-boot headaches. If you simply want to try different desktop environments, these are not tied to whole distributions; you can install as many desktop environments as you like within one distribution and switch between them by logging out and back in again. You can even run different desktop environments simultaneously in different VTs, as described in this question and its answer.
