I'm doing a 72 hr fast (coffee only) but i workout 5 days week typically burning 1000 cals per session. is this still ok to do whilst fasting?

Potentially: There are a few issues. You would certainly want to make sure you are well hydrated throughout the fast, and then obviously your caloric intake will be markedly reduced. The safest thing to do would be to cut back to a less rigorous amount if exercise, and then once it is clear you can tolerate that level without sequela, gradually increase as tolerated. A medical checkup is advisable as well.