if you are planting and keeping them indoors, then you can sow parsely seed anytime, I tried to buy some parsley from a garden centre a few weeks ago and they either didn't have any or what they did have was a tiny plant for big money, so I bought some seed and it is now a very reasonable size on my kitchen window sill along with my basil and chives which I set at the same time. Again with the coriander, if you have the seed, normal compost will be fine indoors, if it is a multipurpose compost, they have a tendency to dry out very quickly and are a devil to get thoroughly wet again, I tend to buy a John Innes one now which is a soil based one, I'm not particular about buying special seed compost although I know some people who swear by it, it's just a little finer, but my plants still come through successfully without using it. Good luck and I hope this helps :)