It depends on what you want to do.
Do you need more disk space for one OS?
Do you want a vfat partition which all OSes can write to?
Can arch and slackware share the same swap partition?
How many drives (for example C
do you want for Windows?
How many partitions (for example / or /home) do you want for arch?
How many partitions do you want for slackware?
Or a more basic question...is all this confusing and you just
want a simple plan that will work?