debian on ext3
 

I already have one ext3 partition unused and one linux swap partition, can these be used to install debian.

Yes,
But it's not recommended.
I'd recommend using a separate partition for:
/ /home /boot /usr /var
Home keeps your user's files.. so allocate as much space as you want..
Usr is like your Program Files folder.
/var is logs, and apace.. allocate a decent amt. of space
/ is your main folder, give it at least 200-250

The idea behind is, if you get errors or so on, you'll usually only corrupt one filesystem. Also if you reinstall your distor, you can still keep your /home separate, and not have to loose everything. /boot separate is for secuirty (you can mount it as read only)