I would install windows first, then slack and finally freebsd. FreeBSD can only be installed on a primary partition, so make sure you have a primary partition for Windows and FreeBSD and either a primary partition for Slack or use an extended partition with a few logical partitions in it for Slack. If you use grub, the entry for FreeBSD will be something like this
Code:
title FreeBSD
root (hd1,2,a)
kernel /boot/loader
hd1 means its on /dev/hdb. The "2" means that its on the third primary partition on /dev/hdb and the "a" means the root partition is located on the first slice of the third partition on /dev/hdb. If this is all jargon to you, then take a look at the manual on the freebsd website.