How many doses of lithium can I safely miss I have not taken for 3 nights as ran out I will start?

If you miss your doses the only problem is that the levels in your system go down and will take time to build up to the therapeutic levels again. So if you haven't had any side effects, or problems with symptoms returning, when you haven't taken the doses, it should be okay. Just start taking it again as per normal ... but let your doctor know that you missed a few doses so that he/she can perhaps get blood levels checked.