You can do any 3/4 without even having done the 1/2, so you definitely COULD do it.
However, while I don't have all the information, you clearly have a chance to redo 1/2 and get yourself on better grounds before continuing on to 3/4. With a score under 65% (I'd even extend it to 70%) in methods, I would suggest that if you can redo 1/2, you should. You might be sitting there thinking, "I don't like maths, I only need a 25 in methods anyway to do my course/I don't want to get a 40 score anyway/[insert other comment tending to not wanting to get an A+]", but under 65 I personally believe would extend to a study score under 25 if you continued the same habits and same scores into 3/4.
Methods is important in the idea that if you get over 30, you've met the pre-requisites for every course in terms of mathematics. The issue is, if you go on as you are now, I can't see you getting over 25, and you never know what the future might hold, regardless of what you think now.