[jambop]

I started off some cabbage, sprout and broccoli to over winter and the have come through very quickly... how long should I wait before thinning into 3" pots ?

[sunshineband]

If they are quite close together I would thin them as soon as they have their first proper leaves, and plant them deeply (up to their seed leaves) into the 3ins pots

If they are well spaced you could leave them until they have a couple of sets of proper leaves

[victoria park]

I take the view that no seedling likes being transplanted ( excepting possibly tomatoes up to their necks ), it just happens to be our necessity with some, especially brassicas. Best to get it over and done with as soon as possible without stem/root damage. Handy things seed leaves. No plant likes to get confused later on.