Silver 3 Pence

Daniel,
There are no mintmarks for these coins.
If you have a 1924, 1925, 1930 halfcrown, keep it, it's a scarce coin, particularly in VF or better condition.

I have an especial regard for silver 3 pence because they were always a treat to get in Xmas pudding. I've not seen them anywhere at less than 40p each, and I suggest you hang on to them.

Other than that, if the coins are in Fine or worse condition, consider scrapping.

For values, and a lot of other info use
http://www.coins-of-the-uk.co.uk/values/index.html

Also buy British Coins Market Values - next edition out about October. (The 2012 edition was priced at £10.99, though available some places at a slight discount).

Don't scrap! Send all your 1917-20 sixpence s to me! Lol but seriously, most of the are worth just melt.