I'm a couple of months away from submitting my thesis but I can relate to how you are feeling. Three weeks is still plenty of time. Read it and make notes of possible questions you may get asked. Also could you even have a practise in front of your supervisors? I know its difficult esp when you have a young child but even set yourself a timetable and try to work around it. I dont know what you are studying but you should take into account that no one will know your thesis better than you, and if your supervisors didnt think your thesis was good enough and that if they thought you couldnt pass your viva, they wouldnt let you submit. Good luck!! :-)