You can always mount your third partition somewhere (like
/mnt/combo or something), and then bind-mount subdirectories from this mountpoint to the three designated directories.
fstab, this would look something like
UUID=... /mnt/combo auto defaults
/mnt/combo/usr /home none bind
/mnt/combo/var /var none bind
/mnt/combo/home /home none bind
Also consider this:
/home makes sense to live on a separate partition - even better, a separate drive, which can be somehow protected (raid, backups,...).
/var would make sense to be separate if you really have something personal in there (websites and such), otherwise it makes no difference.
/usr can definitely be part of
/, it makes no sense to have it separate because on a modern system, the distinction between
/usr/bin is blurred and noone cares about it anymore, and segmenting a system only creates problems if one of the partitions somehow doesn't mount.
/tmp should normally be ram-backed anyway (
tmpfs), unless you really are running out of RAM, and most distros do that by default unless you change it.
Big picture: separate
/home if you have to, the rest is just overhead - you probably have no reason to have different filesystem types or different permissions on any of these, and partitioning doesn't usually mean physical separation (same hard drive?).