1) is a Euphorbia, it's hard to tell the size of it, there are lots, some weeds, some valuable garden plants. If it's about 3 inches high it could be sun spurge - a weed.
2) is a sycamore seedling - I'd get rid of it, there are too many sycamores shedding seed everywhere already.
3) Dunno

I don't know the name of the bottom one either, but what I do know is, it grows in places where drainage is poor and is a pig to get rid of once it takes hold - get it out asap

the third one is definately a weed, had some trouble with one growing in to my garden from my neighbours but weed killer does the trick if its not near any plants.

I've had another look at no 3 - could it be dog's mercury? Glad it's not just me who doesn't know what it is. I almost wrote "weed!" but felt I should have some reason, other than not liking the look of it.

I've just looked at some pictures of dog's mercury and it could well be that. The photo was actually of a clump next door, but a few small ones have now appeared in my garden - and it sounds like I'm best to get rid of them while they're still small!
The euphorbia is about 5 inches high.
Thanks for the help :)

Best advice is get rid of all of them, especially the first one before it takes you over!