Hence means from here, from now, or thus.
- Two years hence this entire episode will be forgotten by all.
- It is going to rain, hence the high humidity and clouds.
Thence means from that time or from that place.
- We flew to Raleigh and thence to Dallas on our way back.
Whence (an old word) means from what place, source, or cause.
- Whence came all this valuable information?