I wear a freestyle libre 3 sensor to monitor my blood sugar. today is my first day. is it okay to exercise, stretch, and lift weights with it in ?

yes, there is no problem, only kkep attention it keeps dry, it is helpful to know your blood sugar levels or glycemia after workout

It should be fine to do all the mentioned activities with the sensor on but you may want to secure it adequately with wrap or other means as depending on how much you sweat and how hairy you are it may otherwise become loose and detach during vigorous exercise. Also if you have diabetes do track your glucose levels during the exercise session and adjust your exercise duration and intensity.