I already have Windows 10 and ubuntu dual booted in my laptop and I would like to triple boot it with El Capitan.
Is it safe? And will it affect other OSes?
I already have Windows 10 and ubuntu dual booted in my laptop and I would like to triple boot it with El Capitan.
Is it safe? And will it affect other OSes?
Sure it is! Grub is not limited to only 2 operating systems. As as long as you have readable partitions that have OSes that grub can detect, it will work just fine.
I personally have 3 Operating Systems; Ubuntu 16.04, Ubuntu Gnome 14.04 and Windows 10.
To answer your second question, no, it will not affect the other OSes. For one, Windows cannot read EXT partitions out of the box. You need to install software which can read EXT in order to access those partitions. At this point, you're doing this on your own.
Linux can access the partitions from other OSes, even the Windows NTFS partitions. However, this where labelling your partitions and knowing what is what is key.
By default, a Linux OS itself will not ever mess with other partitions at all. You can manually access other partitions if you mount them, but again, at this point, you need to be aware that you're browsing the files of another OS and tread carefully. Read all your want, but be aware when you're writing or deleting.