Can I snip the end of leaves at 3 weeks?

You could if you are really bothered by it, but I would leave it alone, it is not harming your plant in any way and to injure your plant unnecessarily by cutting part of it off is not really required. Any part of the leaf you remove will cause a loss of potential energy production and your plant will reuse any matter from damaged leaves for new growth elsewhere. I would suggest transplanting your seedling very soon, there is not much room for root growth in a half filled flower pot of that size. Remember too, this is the most vital time for your plants' establishment for future health and limiting root growth in such a little half filled pot is not in the best interest of your plant. To see the exposed root system of a 3 week old seedling, go to my diary "cannabis conundrums" and in week 7 you will find pictures of the roots and this may give you an idea as to why a decent amount of room is needed and quite early on too. There are also many other pictures of some of the other weird and wonderful things cannabis plants are capable of, as well as pictures of male and female primordia and some hermaphroditic flowers that may also interest any grower who is nervous about what might happen once flowering is about to start. Hope this helps,....... Organoman.