Please help me identify these tall green plants they are now 3 ft high
Well they look like Impatiens to me - can't see the detail of the flower, but the leaves are certainly like Impatiens walleriana, though there are other varieties. These seem a little taller than usual though, and they don't usually pop up on their own from seed, though it depends where you are to an extent. Is it possible you had some planted in that pot last year? Because if you did and they survived the winter, that would explain the increased size. Link to image (it's the first picture) here https://uk.pinterest.com/pin/562035228476275270/ They might be Impatiens New Guinea hybrids - they are taller.
If that's what they are, you can collect the seed, but when the seed pods form, you have to prepare to collect the seed before the pod explodes and shoots its contents all over the place - instructions here http://homeguides.sfgate.com/seeds-impatiens-flower-21048.html
Seeing an open flower clearly, and the seed pods as they form, would help to confirm or deny this ID too.