1
I feel weak today.
Sign 'I', 'feel', and then sign 'weak' by planting your dominant fingertips on your open non-dominant palm and bending them twice. Add a tired facial expression to show that you are physically drained.
2
My legs are weak after the run.
Point to your legs, then perform the sign for 'weak' to show that your muscles are tired and giving out. The visual nature of the sign perfectly matches the feeling of weak legs.
3
That coffee is too weak.
Sign 'coffee', 'too', and 'weak'. While the sign visually represents physical weakness, it is also widely used in ASL to describe things that lack strength or potency, like a weak beverage or a weak excuse.